WASHINGTON — The Justice Department will “leave no stone unturned” to locate five months’ worth of missing text messages from an FBI agent who was removed last summer from special counsel Robert Mueller’s investigative team, Attorney General Jeff Sessions said Monday.

The department last month began providing lawmakers with copies of text communications to and from the veteran counterintelligence agent, Peter Strzok, who was reassigned from Mueller’s Russia investigation following the discovery of anti-Trump messages he had traded by phone with an FBI lawyer.

The department on Friday gave additional text messages to congressional committees, which had requested copies of communications over a two-year period ending last July. But a letter accompanying that delivery revealed that the FBI’s technical system for retaining text messages on bureau phones had failed to preserve communications between Dec. 14, 2016, and May 17, 2017. The latter date is when Mueller was appointed as special counsel to investigate potential coordination between Russia and the Trump campaign.

The explanation for the gap was “misconfiguration issues related to rollouts, provisioning, and software upgrades that conflicted with the FBI’s collection capabilities.”

Sessions said the Justice Department 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.”