Equifax learned about a major breach of its computer systems in March — almost five months before the date it has publicly disclosed, according to three people familiar with the situation.

The company said the March breach was not related to the hack that exposed the personal and financial data on 143 million U.S. consumers, but one of the people said the breaches involve the same intruders.

Either way, the revelation that the 118-year-old credit-reporting agency suffered two major incidents in the span of a few months adds to a mounting crisis at the company, which is the subject of multiple investigations and announced the retirement of two of its top security executives on Friday.

New scrutiny

Equifax hired the security firm Mandiant on both occasions and may have believed it had the initial breach under control, only to have to bring the investigators back when it detected suspicious activity again on July 29, two of the people said.

Equifax’s hiring of Mandiant the first time was unrelated to the July 29 incident, the company spokesperson said. Vitor De Souza, senior vice president for global marketing at FireEye Inc., Mandiant’s parent company, declined to comment.

The revelation of a March breach will complicate the company’s efforts to explain a series of unusual stock sales by Equifax executives. On Aug. 1 and Aug. 2, regulatory filings show that three senior Equifax executives sold shares worth almost $1.8 million, with none of the filings listing the transactions as being part of scheduled 10b5-1 trading plans.

If it’s shown that those executives did so with the knowledge that either or both breaches could damage the company, they could be vulnerable to charges of insider trading.

The U.S. Justice Department has opened a criminal investigation into the stock sales, according to people familiar with the probe.

Equifax has said the executives had no knowledge that an intrusion had occurred when the transactions were made. The company’s shares rose .52 percent in trading Tuesday, closing at $94.87.

New questions about Equifax’s timeline are also likely to become central to the crush of lawsuits being filed against the Atlanta-based company. Investigators and consumers alike want to know how a trusted custodian of so many Americans’ private data could let hackers gain access to the most important details of financial identity, including social security and driver’s license numbers, and steal credit card numbers.

Notifications in March

In statements since disclosing the intrusion on Sept. 7, Equifax said it became aware of the breach only after the data taken by the hackers had been gone for months.

It said it discovered the incident on July 29 and “acted immediately to stop the intrusion and conduct a forensic review.”

Equifax hired Mandiant to help on Aug. 2, and said the investigators eventually learned that the hackers had accessed the data in mid-May.

There’s no evidence that the publicly disclosed chronology is inaccurate, but it leaves out a set of key events that began earlier this spring, the people familiar with the probe said.

In early March, they said, Equifax began notifying a small number of outsiders and banking customers that it had suffered a breach and was bringing in a security firm to help investigate.

The company’s outside counsel, Atlanta-based law firm King & Spalding, first engaged Mandiant at about that time.

While it’s not clear how long the Mandiant and Equifax security teams conducted that probe, one person said there are indications it began to wrap up in May. Equifax has yet to disclose that March breach to the public.