The US Secret Service, the law enforcement agency that protects the president, deleted agents' text messages sent during the January 6 Capitol riot, a government watchdog said in a letter published Thursday.

Joseph Cuffari, the inspector general of the Department of Homeland Security, told Congress in the letter dated Wednesday that his office has had difficulties obtaining records from the Secret Service from January 5 and 6, 2021.

The messages could be crucial to the House of Representatives and Justice Department investigations into whether Donald Trump and his close advisors encouraged the deadly insurrection by the former president's supporters at the US Capitol, which aimed to prevent the certification of Democratic rival Joe Biden as the winner of the November 2020 election.

Secret Service agents were with Trump during the day of the uprising, and were also with vice president Mike Pence, who went into hiding at the Capitol after pro-Trump rioters called for him to be hanged.

On June 29 a former White House staffer told the House January 6 investigation that Trump had attempted to force the Secret Service to take him to the Capitol to join his supporters on that day.

"The Department notified us that many US Secret Service (USSS) text messages, from January 5 and 6, 2021, were erased as part of a device replacement program," Cuffari wrote in the letter first reported by The Intercept and later published by Politico.

"The USSS erased those text messages after OIG requested records of electronic communications" for a review of January 6, he said, referring to the Office of the Inspector General.

In addition, he said, the department has stalled on providing other records to the OIG.

In a statement, Secret Service spokesman Anthony Guglielmi rejected the inspector general's allegation.

The Secret Service is the law enforcment agency responsible for protecting the president of the United States
The Secret Service is the law enforcment agency responsible for protecting the president of the United States AFP / SAUL LOEB

He said the agents' phones were being wiped as part of a planned replacement program that began before the OIG requested the information six weeks after the insurrection.

"The Secret Service notified DHS OIG of the loss of certain phones' data, but confirmed to OIG that none of the texts it was seeking had been lost in the migration," he said.

Cuffari's letter was addressed to the leaders of the Senate and House Homeland Security Committees.

The chairman of the House Homeland Security committee is Representative Bennie Thompson, who is also the chairman of the House committee investigating January 6.

Their investigation has sought to show that Trump knowingly incited the insurrection as an attempted "coup."

The Secret Service has been criticized for not adequately anticipating the threat of the violent action by armed Trump supporters on January 6.

Trump had made a senior Secret Service official at the time, Tony Ornato, his personal deputy chief of staff.

Ornato has denied the account given to the January 6 committee by former Trump aide Cassidy Hutchinson that Trump tried to force the Secret Service to drive him to the Capitol as his supporters massed at the building, the seat of the US legislature.

But other then-White House officials have backed Hutchinson's story.