(CNN) Attorney General Jeff Sessions pledged Monday to get to the bottom of a question asked this week by a growing number of top Republicans: How did the FBI lose a series of messages sent during a critical time period between two bureau employees currently in the crosshairs of an internal Justice Department investigation?

The gap in the exchange -- between top FBI officials who have come under fire for criticizing then-presidential candidate Donald Trump -- was revealed to lawmakers by the Justice Department as it produced a second batch of the pair's text messages to six congressional committees over the weekend.

Senate Homeland Security Committee Chairman Ron Johnson first raised an alarm over the missing messages in a letter sent to FBI Director Christopher Wray on Saturday, and late Monday was joined by the GOP chairmen of three powerful House committees.

"The omission of text messages between December 2016 and May 2017, a critical gap encompassing the FBI's Russia investigation, is equally concerning," House Judiciary Committee Chairman Bob Goodlatte, House Oversight Committee Chairman Trey Gowdy and House Intelligence Committee Chairman Devin Nunes wrote in a joint statement. "Rather than clearing up prior FBI and DOJ actions, these recently produced documents cause us to further question the credibility and objectivity of certain officials at the FBI."

In a cover letter accompanying the Friday delivery of the 384 pages of new text messages, Assistant Attorney General Stephen Boyd explained that technical issues with the FBI's retention software prevented the bureau from capturing messages sent between the two employees on their agency-issued phones from December 14, 2016 to May 17, 2017.

In a footnote, however, Boyd acknowledges an unexplained "discrepancy."

The employees, former top FBI counterintelligence expert Peter Strzok and Lisa Page, an agency attorney, both served briefly on special counsel Robert Mueller's investigation into Russian interference in the 2016 US election.

Their texting history, which Sessions said included over 50,000 messages outside of the uncollected period, is currently under review by the DOJ's inspector general, who is conducting a separate probe of the FBI's handling of the Hillary Clinton email investigation.

In a statement Monday evening, Sessions said he has spoken with the inspector general "to ascertain what occurred" with the missing messages "and to determine if these records can be recovered in any other way."

"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," Sessions said. "If any wrongdoing were to be found to have caused this gap, appropriate legal disciplinary action measures will be taken."

Trump himself weighed in on Twitter Tuesday morning, calling the matter "one of the biggest stories in a long time."

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! — Donald J. Trump (@realDonaldTrump) January 23, 2018

Key player in Clinton probe

Strzok helped lead the FBI's probe of Clinton's private email server and was involved in opening the investigation into the 2016 election meddling. He was reassigned to the FBI's human resources office after the discovery of the messages, which, according to copies viewed by CNN, included insults lobbed at politicians on both sides of the aisle.

Page was also briefly on Mueller's team before returning to the FBI, but she completed her detail before the special counsel's office was made aware of the texts.

In the cover letter, Boyd writes that "misconfiguration issues related to rollouts, provisioning, and software upgrades that conflicted with the FBI's collection capabilities" prevented the bureau-issued Samsung 5 cellphones from capturing or storing text messages, and lays out a timeline of the dates when the two officials' phones stopped storing the messages, and when they received new devices without the technical problems.

But, according to Boyd, "although FBI identified May 22, 2017 as the issued date for Ms. Page's phone," a new Samsung 7 without the storage glitch, collection of her text messages resumed on May 18, 2017.

"The FBI has not yet been able to account for this discrepancy," Boyd wrote.

May 17, 2017, the final day that the pair's messages were not captured by the FBI's system, was also the day when Deputy Attorney General Rod Rosenstein first appointed Mueller as special counsel.

In addition to the missing texts, Republicans are also seizing on the content of the messages that were turned over Friday.

In their statement, Goodlatte, Gowdy and Nunes said that they had met over the weekend to discuss the latest batch of messages and called them "extremely troubling in terms of when certain key decisions were made by the Department of Justice and the FBI, by whom these decisions were made, and the evident bias exhibited by those in charge of the investigation."

In one February 2016 exchange between the two disclosed in the recent batch and outlined in Johnson's letter, Page says it is "unbelievable" that the 2016 presidential race would come down to Clinton versus Trump. Strzok responds, "Now the pressure really starts to finish MYE . . . ," an apparent reference to "Midyear Exam" -- the FBI's internal case name for the Clinton investigation.

In another exchange highlighted by Johnson, Strzok bemoans the timing of news that then-Attorney General Loretta Lynch had decided to accept the recommendations of career supervisors in the DOJ and FBI on the Clinton email investigation, just days after Lynch met privately with former President Bill Clinton aboard her plane in Phoenix, Arizona, texting, "(t)iming looks like hell. Will appear to be choreographed." Page eventually texts back, "And yeah, it's a real profile in couragw (sic), since she knows no charges will be brought."

A first tranche of messages between the two officials that was released last month ignited a political firestorm and fueled a Republican-led effort to discredit the FBI and Mueller's probe.

At FBI and DOJ oversight hearings held shortly after the release of the first messages, Republican congressmen took turns criticizing the agencies for having an alleged bias. Some went as far as to call for the firing of the special counsel.