The congressional committee investigating the US Capitol riot issued subpoenas Tuesday to Rudy Giuliani -- a key figure in Donald Trump's failed bid to overturn the 2020 election -- and three other allies of the defeated ex-president.

They are the latest in a series of demands for testimony and documents sent to members of Trump's inner circle by the House panel, which has shifted its focus to those at the heart of Trump's campaign to cling to power through non-democratic means.

Giuliani led efforts to spread Trump's false voter fraud conspiracy theories after the presidential election, in a bid to challenge results that showed Joe Biden had won fairly.

Giuliani, a once widely-respected former mayor of New York, saw his reputation tarnished by a raft of bizarre media and court appearances in which he spread wild, unsupported allegations of malfeasance.

He was assisted in the disinformation campaign -- which alleged falsely that the election was stolen by the Democrats -- by attorney Jenna Ellis, who has also been subpoenaed.

Rudy Giuliani has seen his standing diminished by a series of bizarre media appearances
Rudy Giuliani has seen his standing diminished by a series of bizarre media appearances GETTY IMAGES NORTH AMERICA via AFP / CHIP SOMODEVILLA

Sidney Powell, another spreader of Trump's election lies, and Boris Epshteyn, an ally of the former president, were the other two subpoena targets.

"The four individuals we've subpoenaed today advanced unsupported theories about election fraud, pushed efforts to overturn the election results, or were in direct contact with the former president about attempts to stop the counting of electoral votes," panel chairman Bennie Thompson said in a statement.

"We expect these individuals to join the nearly 400 witnesses who have spoken with the select committee as the committee works to get answers for the American people about the violent attack on our democracy."

Congressional investigators are probing the violent attack a year ago on the Capitol by supporters of then-president Trump.

The committee is looking into how the attack, which shut down the US Congress, came together and whether Trump and members of his circle had any part in encouraging it.

It has already issued subpoenas for several key figures in Trump's orbit, including one-time White House strategist Steve Bannon and chief of staff Mark Meadows.