Senator Heidi Heitkamp (D-ND) speaks next to Sen. Susan Collins (R-ME) at a news conference with a bipartisan group of senators on Capitol Hill in Washington, D.C., U.S., to unveil a compromise proposal on gun control measures, June 21, 2016. REUTERS/Yuri Gripas

By Pete Schroeder

WASHINGTON (Reuters) - Thirty-six U.S. senators on Tuesday called on federal authorities to probe the sale of nearly $2 million in shares of credit bureau Equifax Inc by company executives after a massive data breach this summer, and one compared their actions to insider trading.

The lawmakers signed a letter asking the Justice Department, Securities and Exchange Commission and Federal Trade Commission to probe about $1.8 million in stock sales by three executives between the time the company learned about the hack and when they made it public.

"If that happened, somebody needs to go to jail," Senator Heidi Heitkamp, a Democrat on the Senate Banking Committee, said at a credit union industry conference in Washington. "It's a problem when people can act with impunity with no consequences. How is that not insider trading?"

Cyber security experts believe the breach is one of the largest data hacks ever disclosed.

On Wall Street on Tuesday, Equifax closed 2.5 percent higher at $115.96, slightly reversing a 21 percent slide since the hack was reported.

In their letter, the lawmakers, led by Jack Reed, a Democrat from Rhode Island, and John Kennedy, a Louisiana Republican, requested "a thorough examination of any unusual trading, including any atypical options trading, for violations of insider trading law."

"We request that you spare no effort in your investigations and in enforcing the law to the fullest extent," the lawmakers said.

The Federal Bureau of Investigation has already said it was investigating the Equifax hack. A spokesman for SEC Chairman Jay Clayton declined to comment.

The acting FTC chairman, Maureen Ohlhausen, declined to say if that agency was investigating the breach. "We're trying to get a handle on the scope of all of this. We're certainly taking this very seriously," she told reporters at an antitrust conference.

The agency has historically probed big breaches but only sued companies that had been sloppy in protecting consumer data.

The breach also prompted expressions of concern from U.S. Treasury Secretary Steven Mnuchin, while Massachusetts' attorney general said her state planned to sue.

Equifax announced last week that it had learned on July 29 that hackers infiltrated its systems in mid-May. The hackers pilfered names, birthdays and addresses, as well as Social Security and driver’s license numbers - a treasure trove for identity thieves. Data of up to 143 million people may have been exposed.

Following revelations of the stock sales by Chief Financial Officer John Gamble and two other people, Equifax said in a statement last week that the executives were not aware that an intrusion had occurred when they sold their shares.

Massachusetts Attorney General Maura Healey said she intended to file a lawsuit against Equifax for allegedly failing to maintain appropriate safeguards to protect customers' data, including that of nearly three million Massachusetts residents.

"In all of our years investigating data breaches, this may be the most brazen failure to protect consumer data we have ever seen," Healey said in a statement.

Equifax had initially appeared to offer credit monitoring to breach victims only if they forfeited their right to file a lawsuit. New York Attorney General A.G. Schneiderman said Tuesday that Equifax removed that language from its website.

U.S. Treasury Secretary Steve Mnuchin on Tuesday called the Equifax breach "quite unfortunate" and insisted that his top priority is to make sure financial data is safe.

"I am concerned about the global financial system and keeping it safe," he said at the CNBC Institutional Investor Delivering Alpha Conference in New York.

Equifax did not respond to Reuters requests for comment.

(Reporting by Pete Schroeder. Additional reporting by Diane Bartz and Sarah N. Lynch; editing by Dan Grebler and Chris Sanders)