Fascinating that there is always more to the story. Remember, this was/is confidential and personal data. Further, Alibaba is a Chinese international holding company that is a counterpart to Amazon and specializes in artificial intelligence based in Hangzhou, China.

The General Accounting Office issued a report on Equifax. The GAO analysis detailed the steps Atlanta-based Equifax has taken since the breach to prevent similar attacks in the future. Last year, hackers had found a vulnerability in Equifax servers that gave them access to customer login credentials.

The report said the hackers hid in Equifax’s system for more than two months and mined data for credit card numbers, drivers licenses and social security numbers. The breach led the agency to make $200 million in security upgrades.

WSJ: Two years before Equifax Inc. stunned the world with the announcement it had been hacked, the credit-reporting company believed it was the victim of another theft, only this time at the hands of Chinese spies, according to people familiar with the matter.

In the previously undisclosed incident, security officials feared that former employees had removed thousands of pages of proprietary information before leaving and heading to jobs in China. Materials included code for planned new products, human-resources files and manuals.

Equifax went to the Federal Bureau of Investigation and the Central Intelligence Agency. Investigators from the company and the FBI came to view events at Equifax as potentially a huge theft of data—not of consumers’ personal data, as happened with the subsequent 2017 hacking of Equifax’s files, but of confidential business information.

Equifax security officials briefed the then-chief executive, Richard Smith, at a fall 2015 meeting, spreading high stacks of paper across the length of the boardroom table. The voluminous printouts represented what they feared was stolen. Adding to suspicions, the Chinese government had recently asked eight companies to help it build a national credit-reporting system.

At one point, Equifax grew so worried it began building a way to monitor the computer activity of all of its ethnic-Chinese employees, according to people familiar with the investigation. The resource-heavy project, which raised legal concerns internally, was short-lived.

Some investigators believed Equifax’s intense focus on the matter contributed to a delay in the company’s understanding the extent of the 2017 hack of consumers’ information, an event that hammered Equifax’s stock, cost some executives their jobs, including Mr. Smith, and damaged the company’s reputation.

Ultimately, the previously undisclosed investigation undertaken by the FBI stalled. The FBI wanted to pursue a criminal case, believing the theft of trade secrets costs the U.S. hundreds of billions of dollars a year, with China the leading offender, said people familiar with the investigation. Equifax began to worry about legal exposure and how onerous the inquiry could become, according to these people, and eventually reduced its cooperation with law enforcement.

That left many of the questions raised by the investigation, both about Equifax and about China, unresolved.

This account of the events at Equifax is based on people familiar with the investigation.

Equifax, in a written statement, said it became aware in 2015 of “efforts by a former employee to obtain company information, and launched an internal investigation into his activities.” The company “brought the investigation to the attention of U.S. law enforcement authorities and cooperated with the federal agencies,” Equifax said.

“Although this individual had improperly obtained proprietary Equifax information,” the statement said, “the information we determined was accessed was general in nature and not material or harmful to Equifax, consumers or our business clients.” Equifax said the company has “no evidence to suggest that consumer data or other personal information was compromised, or that this individual targeted this type of information.”

Equifax didn’t address in its statement whether it thought other employees were involved. A person familiar with the company’s thinking disputed the notion that Equifax reduced its cooperation with law enforcement in a probe it had itself triggered.

Representatives of the FBI and CIA declined to comment. The Chinese Embassy in Washington didn’t respond to requests for comment.

One of the former employees Equifax and the FBI investigated in connection with a possible business-information theft was Daniel Zou, who worked in Toronto. The company he joined in China was Ant Financial, a fast-growing financial-technology affiliate of Alibaba Group Holding Ltd. , founded by billionaire Jack Ma.

Both Ant and Mr. Zou denied any involvement in taking proprietary Equifax data. Alibaba referred questions to Ant.

Ant, based in Hangzhou, China, said it “has never used Equifax code, scripts or algorithms in the development of its own products and services.”

Mr. Zou, in a sworn statement provided by his lawyer, said, “I deny that I worked with or consulted with a network of Equifax colleagues to steal Equifax code for Ant Financial or that I provided any such code to Ant Financial.”

Interviewed by The Wall Street Journal in Washington, Mr. Zou, a 35-year-old Chinese-born Canadian citizen who graduated from the University of Toronto, repeated his denial and said that learning from the Journal of Equifax’s suspicions had been “a nightmare.”

Those suspicions arose in 2015, a few months after Mr. Zou left his job as an Equifax product manager to join Ant’s new credit-scoring business, which is known as Sesame Credit in English. Ant was among the companies asked by China’s central bank to develop credit-scoring services. Sesame launched its service in January 2015, several months before Mr. Zou came aboard.

Equifax’s data-loss prevention system, which guards against sensitive information leaving the corporate network, flagged the activities of Mr. Zou, according to people familiar with the investigation. The system alerted that an employee might have taken data off the network, and initially registered it as benign, they said.

Mr. Zou said in his interview with the Journal that, according to his understanding of how the system works, it would warn the person removing the data on the spot. He said he never received such a warning. Equifax declined to say whether that is how the system works or whether Mr. Zou received a warning.

At the same time, Equifax officials also had suspicions about a different employee, in another city. Equifax’s security chief, Susan Mauldin, approached the FBI with a question: What would it look like if we were being targeted by China?

FBI officials told her that in one common technique, a group makes plans to visit a company’s office to pitch a partnership, then at the last minute replaces delegation members with spies.

Around this time, a delegation from a Chinese business visited Equifax and swapped out some members at the last minute, fueling Equifax’s suspicions it was a target.

Company security officials decided to examine Mr. Zou’s computer activity. They discovered he had printed out thousands of pages of company information. The material related to the way credit scores are obtained, what different pieces of data mean and how to apply algorithms to assess troves of data, according to the people familiar with the investigation. They said some was information that could help explain products Equifax was working on.

At around the same time they were examining Mr. Zou’s systems, investigators discovered what they believed to be a major infiltration campaign. They found that other employees had sent code to their personal email accounts and uploaded it to software-development platforms others could access.

According to the people familiar with the probe, the investigators, by talking to Equifax employees and examining email accounts and LinkedIn messages sent to them, saw indications that recruiters purporting to represent Ant affiliate Alibaba had offered to triple salaries for certain ethnically Chinese Equifax employees—and provided instructions on specific Equifax information they should bring along if they jumped ship.

The investigators saw, as well, that Mr. Zou had searched the Equifax human-resources system to look up data analytics teams in the U.S. He had printed out contact information for many ethnic-Chinese employees, according to people familiar with the probe. They said some of those employees told colleagues they were later contacted by recruiters who claimed to be working on behalf of Alibaba.

The investigators found notes on Chinese messaging service WeChat in which another group of Equifax employees in North America, using their company-issued phones, arranged off-hours meetings to discuss work projects and left the company soon after, saying they were going to Ant or Sesame for big raises.

Ant said Mr. Zou is the only former Equifax employee it has hired since it began collecting employment history information in 2011. Ant said Mr. Zou began at its credit-scoring business in May 2015. It listed a five-figure starting salary for Mr. Zou and said he wasn’t promised any large bonuses.

Ant said it didn’t “directly or indirectly through third-party recruiters” encourage job applicants to steal Equifax information. Ant prohibits employees and recruiters from requesting such activity, the company said, adding that third-party recruiters aren’t authorized to make job offers on its behalf.

Ant said it hadn’t been contacted by Equifax or any government investigators about such matters. After receiving an inquiry from the Journal about Mr. Zou, Ant said, it investigated his information-technology activities and found no evidence he had ever provided Ant with any Equifax code, scripts or algorithms.

Mr. Zou said he worked in marketing and didn’t have access to Equifax code, algorithms and other proprietary information; never took any to Ant; wasn’t asked to; and never encouraged others to.

“I deny that I searched an internal Equifax human resources database to recruit Equifax employees to join Ant Financial,” Mr. Zou said in the sworn declaration provided by a lawyer. “I further deny that I printed contact information for ethnic-Chinese Equifax employees as part of an effort to recruit such employees to join Ant Financial.”

In the Journal interview, Mr. Zou said, “I think [where] this might come from is that during my time at Equifax I had a habit of sending work-related documents to my own email so that I could work at home. If any of those contain [any] of what they call the alleged proprietary information, right after I left Equifax and before I went back to China, I deleted them all. And I did not share that with anybody.”

If investigators were alarmed by his email practices, Mr. Zou said, “I think that’s a huge misunderstanding.”

Mr. Zou also said he printed out employee contact information for projects that required him to work with global colleagues. “Equifax Canada did not want to reinvent the wheel from beginning,” he said, “so my job was to piggyback the success case” from the company’s U.S., U.K. and Latin American regions.

He said he disposed of all the documents before moving to China and joining Ant, and he denied targeting any ethnicity. “If you search a data analytics team, the likelihood is high that you will reach a Chinese employee,” he said.

Mr. Zou said he had never been contacted by Equifax or any government authorities about data theft, and learning he was suspected caused him “emotional turmoil.”

Although Equifax had gone to the FBI—and although the bureau was eager to pursue the matter—Equifax officials by the middle of 2016 had grown wary of providing more information to federal investigators.

Equifax worried that doing so could trigger requirements under securities law for disclosure of material information, said the people familiar with the investigation. They said Equifax also was concerned that handing over access to its entire network, including international operations, as the FBI had requested, could run afoul of obligations in some countries where Equifax operates.

Around the middle of 2016, Equifax told its internal investigators to comply with any potential subpoenas but to stop proactively providing information to law enforcement, said the people familiar with the investigation.

The person familiar with Equifax who disputed the notion the company directed employees to be uncooperative said: “As the investigation progressed, we did ask that requests for information be passed through our legal office to ensure we were adhering to standard legal protocols.”

Equifax continued to monitor certain employees through 2016 and 2017. It eventually confronted several ethnically Chinese employees over activities found in its investigation, who left before the company took further action, according to people familiar with the probe.

FBI officials in Atlanta got the impression from Equifax’s then-CEO, Mr. Smith, and legal staff that the company didn’t believe it generally had information valuable enough to be the target of a major Chinese campaign.

Mr. Smith told colleagues even if thieves had taken code, they didn’t have Equifax’s consumer data, which meant the theft wouldn’t pose a competitive threat. Moreover, Equifax didn’t see a material impact on current operations because the information that appeared to have been stolen related to products in development, not to existing ones.

The U.S. attorney’s office in Atlanta ultimately determined it didn’t have evidence the suspected thefts were directed by the Chinese government, a top priority for law enforcement. The prosecutors decided they wouldn’t pursue a case against any individual, since Equifax wasn’t eager to do so, and since what former employees were suspected of taking was corporate information, rather than anything directly affecting U.S. consumers.

The U.S. attorney’s office declined to comment.

Then, in September 2017, came blockbuster news from Equifax: the disclosure that a hacking of its files had exposed highly sensitive personal data on more than 140 million Americans.

Equifax had learned six months earlier, in March 2017, of a software vulnerability, but waited months to fully check its encrypted traffic to see whether it had been breached. Only in July 2017 did Equifax realize the hack had exposed personal information, including Social Security numbers and dates of birth, of nearly half the U.S. population.

This delay was partially due to Equifax’s failure to resolve a dispute between its technology and information-security staffs at a time when top security people were focused on possible infiltration from China, in the opinion of some of the people familiar with the investigation.

The person familiar with Equifax’s thinking said the hack involved both human error and technological failure, and Equifax has been forthcoming about the causes.

In the weeks following the disclosure of that giant 2017 breach, Mr. Smith resigned, as did Ms. Mauldin and Equifax’s chief information officer, David Webb. All either couldn’t be reached or didn’t respond to requests for comment.

In January 2018, Chinese officials rolled out a state-backed credit-scoring company and gave Ant Financial an 8% stake.

Mr. Zou has returned to Canada. Ant transferred him from Sesame Credit to its Alipay international business unit in Hangzhou in mid-2017. On June 1 of this year, he moved to Alipay Canada in Vancouver.