The US attorney general, Merrick Garland, has been asked to investigate yet another deletion of text messages and other communications by senior officials on 6 January 2021, this time by the Pentagon.

American Oversight, a non-partisan watchdog group, revealed the shock deletion on Tuesday, having discovered it through freedom of information requests to the Department of Defense.

The DoD and the army admitted in court filings to American Oversight that the phone messages of senior Trump officials were wiped after the administration handover, including text messages from January 6, the day of the deadly attack on Congress by supporters of Donald Trump.

Similar deletions of communications around January 6 by the Department of Homeland Security and the Secret Service were already the subject of considerable controversy.

The Department of Justice and the House January 6 committee continue to investigate Trump’s attempt to overturn his election defeat.

In an open letter to Garland, American Oversight said: “We urge you to investigate DoD’s failure to preserve the text messages of several high-ranking officials on or surrounding the day of the January 6 attack.”

In its Freedom of Information request, American Oversight sought the release of communications between senior officials and Trump, his vice-president, Mike Pence, his chief of staff, Mark Meadows, “or anyone communicating on their behalf on January 6”.

Among officials whose communications are at issue are the former acting defense secretary Chris Miller; former army secretary Ryan McCarthy; Kash Patel, who was Miller’s chief of staff; Paul Ney, formerly Pentagon general counsel; and James E McPherson, formerly general counsel of the army.

The Pentagon’s sluggish response to the Capitol attack remains the subject of widespread speculation and investigation.

As the New York Times put it last month, “the mobilisation and deployment of national guard troops from an armory just two miles away from the Capitol was hung up by confusion, communications breakdowns and concern over the wisdom of dispatching armed soldiers to quell the riot”.

Messages between senior DoD officials and the White House could shed light on what happened.

On Tuesday, Ney told CNN he turned in a phone when he left the Department of Defense on 20 January 2021, the day of Joe Biden’s inauguration.

“I did not wipe the phone before I turned it in (or ever that I can recall),” Ney said. “When I turned the phone in, I did not know what was going to be done with that device nor do I know what actually was done with that device after I turned it in.

“If DoD represented in litigation that the device was wiped after I left DoD on inauguration day, I believe that is very likely what happened and when it happened, but I do not know why.”

On January 6, a mob Trump told to “fight like hell” attacked Congress in an attempt to stop certification of Biden’s election victory. Nine deaths have been linked to the riot, including suicides among law enforcement officers.

In a series of dramatic public hearings, the House January 6 committee has demonstrated Trump’s role in election subversion efforts and in stoking the attack on the Capitol.

Garland is under increasing pressure over investigations surrounding January 6 and Trump’s election subversion. He has promised to “pursue justice without fear or favor”.

Heather Sawyer, executive director of American Oversight, told CNN: “It’s just astounding to believe that [the Pentagon] did not understand the importance of preserving its records – particularly [with regards] to the top officials that might have captured what they were doing, when they were doing it, why they were doing it on that day.”

The deletion of such records, she said, “reveals a widespread lack of taking seriously the obligation to preserve records, to ensure accountability, to ensure accountability to their partners in the legislative branch and to the American people”.