Richard F. Smith, who stepped down last week as Equifax’s chief executive, is scheduled to testify on Tuesday before a congressional subcommittee that is investigating the theft and Equifax’s response to it. It is the first of four congressional hearings he is scheduled to speak at this week.

“I am deeply sorry that this occurred,” Mr. Smith said in prepared remarks released on Monday. “Equifax was entrusted with Americans’ private data and we let them down.”

Paulino do Rego Barros Jr., a longtime Equifax executive who was promoted after Mr. Smith’s departure to serve as the company’s interim chief executive, said that he was advised on Sunday that Mandiant had completed the forensic phase of its investigation and found the additional accounts that were potentially at risk.

“I directed that the results be promptly released,” Mr. Barros said in a statement. “Our priorities are transparency and improving support for consumers.”