Undercover FBI agents were not present during the 2021 attack on the US Capitol by Donald Trump supporters, a Justice Department watchdog said Thursday in a report debunking a popular right-wing conspiracy theory.

"We found no evidence in the materials we reviewed or the testimony we received showing or suggesting that the FBI had undercover employees in the various protest crowds, or at the Capitol, on January 6," Justice Department inspector general Michael Horowitz said in an 88-page report.