Originally Published: 01 AUG 22 15:48 ET

Updated: 01 AUG 22 22:57 ET

By Whitney Wild and Priscilla Alvarez, CNN

(CNN) -- Key House Democrats have issued a new call for the Department of Homeland Security's inspector general to recuse himself from a probe of missing Secret Service text messages after a CNN exclusive report showed investigators knew for more than a year texts had been erased.

House Oversight Committee Chairwoman Carolyn Maloney and House Homeland Security Chairman Bennie Thompson reiterated their call for Inspector General Joseph Cuffari to step aside in a letter on Monday, citing concerns about "your lack of transparency and independence, which appear to be jeopardizing the integrity of a crucial investigation run by your office."

Maloney and Thompson also are demanding transcribed interviews with key DHS IG staffers. CNN first reported that DHS inspector general investigators dropped efforts to recover missing Secret Service text messages in July 2021, a year before Cuffari raised concerns about Secret Service and DHS transparency to congressional oversight committees.

"The Committees have obtained new evidence that your office may have secretly abandoned efforts to collect text messages from the Secret Service more than a year ago," the letter said. "These documents also indicate that your office may have taken steps to cover up the extent of missing records, raising further concerns about your ability to independently and effectively perform your duties as Inspector General (IG)."

The committees are requesting a slate of communications and documents by Monday, ranging from correspondence related to any decisions not to collect or recover text messages to communications related to notifying Congress.

Sen. Dick Durbin, an Illinois Democrat, on Monday reiterated his call for the Justice Department to investigate the missing text messages.

"This is about the destruction of critical evidence, whether it's material to the January 6 episode or not. The fact that this man, Joseph Cuffari, as inspector general, could not get the information that should have been transferred from administration to the other and didn't report it properly to Congress or to the agency that he's working at, we may have jeopardized some very critical evidence when it comes to the historic record on January 6 and he treated it as almost a routine event rather than something that should have been highlighted," Durbin told CNN's Don Lemon.

In a statement to CNN, a spokesperson for the Department of Homeland Security Office of Inspector General said it "does not discuss ongoing administrative reviews and does not confirm the existence of, or otherwise comment about criminal investigations."

Watchdog defends himself

However, in an internal email to employees obtained by the Project on Government Oversight and shared with CNN, Cuffari defended himself and commended them for their work amid an "onslaught of meritless criticism."

"In the past couple of weeks, DHS OIG has been the subject of a tremendous amount of public speculation," Cuffari told staff in an email obtained by the Project on Government Oversight and shared with CNN.

"Because of US Attorney General guidelines and quality standards, we cannot always publicly respond to untruths and false information about our work," he wrote. "I am so proud of the resilience I have witnessed in the face of this onslaught of meritless criticism."

The email, sent at 2:28 p.m. Monday, arrived shortly before key House Democrats accused Cuffari's office of manipulating and omitting information about its investigation into missing Secret Service and top DHS officials' text messages.

The letter shows a DHS deputy inspector general, Thomas Kait, wrote an email to a DHS senior liaison, Jim Crumpacker, on July 27, 2021, advising DHS investigators were no longer seeking text messages. Kait is one of the staffers the committee wants to interview now.

"Jim, please use this email as a reference to our conversation where I said we no longer request phone records and text messages from the USSS [United States Secret Service] relating to the events on January 6th," the email said, according to the letter.

The letter also confirms CNN reporting that the probe into text messages was reopened in December 2021.

Lawmakers said in Monday's letter that Kait also removed "key language" from a February memo to DHS underscoring the significance of text messages to the inspector general's investigation. The original memo mentioned that most DHS components had not provided requested information and noted text message content is a "critical source of information for the DHS OIG review," but the final version stated the opposite, saying that they had received responses, according to the letter.

"These documents raise troubling new concerns that your office not only failed to notify Congress for more than a year that critical evidence in this investigation was missing, but your senior staff deliberately chose not to pursue that evidence and then appear to have taken steps to cover up these failures," the letter states.

It goes on to cite missing text messages for the two top Homeland Security officials under former President Donald Trump -- acting Secretary Chad Wolf and acting deputy secretary Ken Cuccinelli. Information obtained by the committee revealed that the inspector general's office was aware in February that those messages couldn't be accessed but didn't notify Congress. CNN has reached out to Cuccinelli for comment.

Latest twist in saga

Monday's letter is yet another twist in the ongoing saga over missing messages around January 6. Memos obtained by CNN indicate that the Department of Homeland Security repeatedly reminded the workforce to comply with the inspector general and relevant Hill committees.

After the Office of Inspector General raised concerns to Homeland Security Secretary Alejandro Mayorkas about compliance with requests, the secretary issued a September 2021 memo to the workforce saying that employees should cooperate with interviews and provide information.

"The Department is committed to supporting the OIG's mission. DHS employees are expected to cooperate with OIG audits, inspections, investigations, and other inquiries. Any effort to conceal information or obstruct the OIG in carrying out its critical work is against Department directives and can lead to serious consequences," the memo says.

Then, in October 2021, DHS General Counsel Jonathan Meyer issued a memo specific to January 6, 2021, and saying the office was cooperating with the House select committee investigating the Capitol Hill insurrection.

"I am therefore directing the Department and its Components to respond to any Select Committee requests it receives expeditiously and thoroughly," that memo states. "Such cooperation and transparency are vital to the Department's obligation to safeguard our Nation and its foundational democratic principles."