The FBI was unable to preserve some text messages between two agents who have been accused of anti-Trump bias, including one who previously served on special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s team, according to a letter from the bureau to lawmakers.

The FBI said it did not have a record of messages exchanged over a six-month period between Peter Strzok and Lisa Page because many of its issued mobile phones had problems related to “rollouts, provisioning, and software upgrades.”

“The result was that data that should have been automatically collected and retained for long-term storage and retrieval was not collected,” the FBI explained in the letter.

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Sen. Ron Johnson Ronald (Ron) Harold JohnsonThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - White House moves closer to Pelosi on virus relief bill Second GOP senator to quarantine after exposure to coronavirus GOP-led panel to hear from former official who said Burisma was not a factor in US policy MORE (R-Wis.) expressed concerns over the missing messages, according to a letter he sent to FBI Director Christopher Wray, obtained Sunday by The Hill.

Strzok, who also worked on the FBI’s investigation into Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE’s use of a private email server, was removed last summer from Mueller’s probe into possible collusion between the Trump campaign and Russia after it was discovered he had sent anti-Trump messages.

Some Republicans seized on the news as an indication that Mueller’s probe was biased against President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE.

The missing messages were sent between Dec. 14, 2016, and May 17, 2017, according to Johnson’s letter.

The Justice Department on Friday also provided Johnson with 384 pages of additional messages between Strzok and Page, according to the letter.

One exchange, disclosed in Johnson's letter, shows the two agents criticizing the timing of then-Attorney General Loretta Lynch’s announcement that she would accept the FBI’s recommendation on the Clinton email investigation.

Johnson, who chairs the Senate's Homeland Security and Governmental Affairs Committee, asks Wray to explain the scope of the missing messages, and if the FBI has searched Strzok's and Page's non-FBI phones for possible federal records.