WASHINGTON — The Department of Homeland Security’s internal watchdog, who is under criticism for his handling of an investigation into missing Secret Service text messages around the time of the U.S. Capitol attack, is refusing to cooperate with congressional demands, even blocking his employees from testifying before Congress, two top Democrats said Tuesday.

Reps. Carolyn Maloney, D-N.Y., chair of the Oversight Committee; and Bennie Thompson, D-Miss., chair of the Homeland Security Committee, sent a letter to Joseph Cuffari, the Homeland Security inspector general, demanding that his office comply with their requests for documents and transcribed interviews.

“Your obstruction of the committees’ investigations is unacceptable, and your justifications for this noncompliance appear to reflect a fundamental misunderstanding of Congress’ authority and your duties as an inspector general,” the letter said.

The clash is the latest development surrounding missing text messages from around Jan. 6, 2021, that were sent and received by Secret Service agents and later erased. It is unclear what the deleted messages said or how many are missing, but congressional panels are scrutinizing what agents were saying and doing as President Donald Trump insisted on joining a crowd of his supporters at the Capitol after mob violence began that day.

Cuffari, who was appointed by Trump, said the Secret Service hampered his investigation into the matter. Congressional committees accuse him of failing to adequately carry out an inquiry.

Adding to Congress’ concern: deleted text messages related to the Capitol attack from top Homeland Security and Defense Department officials in the Trump administration. Defense officials have said that some phones of Trump administration officials were “wiped” when they left their government jobs.

Cuffari’s office similarly conducted a so-called iPhone refresh in June, when employees were instructed to wipe their devices and decide which text messages should be saved. The inspector general’s office did not respond to questions about whether Cuffari preserved any relevant text messages during the process.

Sen. Gary Peters, D-Mich., chair of the Homeland Security and Governmental Affairs Committee, sent letters last week to Cuffari, the Homeland Security secretary and the departing Secret Service director, requesting more information about the deleted communications and the process for preserving federal records.

Cuffari told Congress last month that Secret Service text messages from Jan. 5 and 6, 2021, had been erased, suggesting that it occurred as part of a device replacement program. He has said those whose messages were missing included agents who were part of Trump’s security detail.

The inspector general also directed the Secret Service to halt its internal search for purged texts so it would not “interfere with an ongoing criminal investigation.”

Maloney and Thompson said Cuffari’s office had delayed telling Congress about the missing messages for months and hampered an internal investigation into the matter. His office “may have taken steps to cover up the extent of missing records,” they said.

The lawmakers have called on Cuffari to recuse himself from the investigation, a demand he has refused. They have also called for two officials in his office to testify.

Maloney and Thompson said Cuffari sent them a letter Aug. 8 “refusing to acknowledge any failures in the adequacy and timeliness” of his notification to Congress.

“You gave no indication that you would step aside from the investigation, raising the prospect that the inquiry could be further compromised,” the lawmakers wrote. “You also refused to provide any documents responsive to our request and/or allow your staff to be interviewed, stating, ‘We do not authorize our staff to sit for transcribed interviews with your committee about these ongoing matters.’ ”

They warned Cuffari they would “consider alternate measures to ensure” compliance. While Maloney and Thompson were not specific about these potential steps, they could subpoena documents and testimony. They gave his office until Aug. 23 to provide “all responsive documents” and make personnel available for interviews before lawmakers issue a congressional subpoena.