The FBI may have ignored its own data storage procedures as it lost five months of text messages between two anti-Trump FBI employees, security analysts said.

Texts that have been made public, which show the two employees and reported lovers talking about their antipathy toward the president and efforts to impeach him, have become a major embarrassment for an FBI already reeling from accusations of bias and bungling.

But the Justice Department said it lost five months of texts from FBI agent Peter Strzok during a key period when he and FBI lawyer Lisa Page were venting about Mr. Trump and investigations surrounding the 2016 presidential election.

Mr. Trump on Tuesday called the missing messages "one of the biggest stories in a long time," and the White House said reporters should treat the lost texts with the "same obsession" they have approached suspicions of collusion between Russia and the presidential campaign.

The FBI couple exchanged more than 50,000 texts from July 1, 2015, to July 28, 2017.

The FBI was expected to provide Congress with a new batch of texts that the couple exchanged from Dec. 14, 2016, to May 17, 2017. But the agency told lawmakers over the weekend that "misconfiguration issues" caused a loss of the texts.

"I can't say that I ever saw anything like this during my time with the FBI," said Don Vilfer, a former agent who heads the computer forensics division at Vand Group LLC. "It's an anomaly that something like this would happen."

Republicans wondered if something more nefarious was at play.

Rep. Lee M. Zeldin, New York Republican, said the government should name a special counsel to investigate the matter.

"We cannot expect the DOJ and FBI to properly investigate themselves with so much significant, mounting evidence of misconduct at the highest levels regarding how and why the Clinton probe ended and the Trump-Russia probe began," Mr. Zeldin said.

Ms. Page and Mr. Strzok were at one time assigned to special counsel Robert Mueller's team investigating links between Russia and the Trump campaign in the run-up to the 2016 election.

The pair ended up at the center of the controversy surrounding the Mueller investigation.

Mr. Strzok referred to the president as "an idiot" in one of the more than 384 texts that the Department of Justice turned over to Congress. Another text cryptically mentions the need for an "insurance policy" should Mr. Trump be elected.

After the ant-Trump texts became public, Mr. Strzok was removed from the Mueller team. Ms. Page left last summer.

Mr. Trump has pushed for attention to be given to the missing messages.

"In one of the biggest stories in a long time, the FBI now says it is missing five months worth of lovers Strzok-Page texts, perhaps 50,000 and all in prime time. Wow!" the president tweeted Tuesday morning.

Ranking House Democrats fired back, accusing Mr. Trump and congressional Republicans of using the missing emails to undermine the Mueller investigation.

Reps. Jerrold Nadler, New York Democrat; Elijah E. Cummings, Maryland Democrat; and Adam B. Schiff, California Democrat, said Republicans want to portray the FBI as anti-Trump, but they claimed the agency's reopening of its investigation into Hillary Clinton's deleted emails just weeks before the election cost her the presidency.

"These Republican attacks show their desperation at the fact that Mueller already has obtained two guilty pleas, two indictments and at least two cooperating witnesses," the statement read.

Sen. Ron Johnson, Wisconsin Republican, countered by saying Mr. Strzok expressed doubt to Ms. Page about the Mueller investigation two days after it began. Appearing on a local Milwaukee radio show, Mr. Johnson said Mr. Strzok was reluctant to join the team because he didn't believe it would find anything incriminating about Mr. Trump.

"Strzok says, quote, 'You and I both know the odds are nothing,'" Mr. Johnson said. "'If I thought it was likely, I'd be there no question. I hesitate in part because of my gut sense and concerned there's no big there there.'"

Mr. Johnson said the text was sent on May 19, two days after Mr. Mueller was appointed as special counsel.

Meanwhile, the fight on Capitol Hill is still brewing over a House Permanent Select Committee on Intelligence memo written by Republicans claiming bias at the FBI and Justice Department, stemming from an anti-Trump dossier.

Lawmakers who have seen the memo say the information is alarming, but some of the material is classified and members are divided on releasing it to the public.

The White House said Tuesday that it will leave the decision to the committee on whether to release the memo.

"We certainly support full transparency," said White House press secretary Sarah Huckabee Sanders. "It sounds like there are some members in the House that have some real concern with what's in that memo and feel very strongly that the American public should be privy to see it."

On Tuesday night, Mr. Johnson, chairman of the Homeland Security and Governmental Affairs Committee, and Sen. Chuck Grassley, Iowa Republican, wrote a letter to the Justice Department inspector general, Michael Horowitz.

The senators asked Mr. Horowitz to explain why he did not disclose the FBI's failure to provide missing texts and what steps the inspector general is taking to investigate how the texts disappeared.

Mr. Horowitz has until Jan. 29 to respond to the letter, the senators said in an announcement.

While politicians debate whether to release the content of the text messages, technology analysts question how five months of emails could have disappeared. Some said it is likely the bureau's procedures were not followed, but further investigation would be needed to determine if that was intentional or accidental.

"This is unusual and contrary to FBI policy, which requires that all records be preserved unless they are specifically cleared for disposal," said Steve Aftergood of the Federation for American Scientists. "Is it the sign of something nefarious? Maybe. But it could also be a sign that the two agents were allegedly having an affair and took great steps to delete their texts."

The FBI did not respond to requests for comment.

On Monday, Attorney General Jeff Sessions vowed to find the missing texts.

"We will leave no stone unturned to confirm with certainty why these text messages are not now available to be produced and will use every technology available to determine whether the missing messages are recoverable from another source," Mr. Sessions said. "If we are successful, we will update the congressional committees immediately."

Mr. Vilfer said he has recovered text messages that had been deleted over a year ago. But he cautioned that recovery is dependent on a number of factors, including the number of times the phone was used and the period between when the message was sent and when the recovery process starts.

"More information is needed before we can say whether or not this was an act of sloppy data handling or something that was intentionally done to get rid of data so it couldn't be found later," he said.