Washington (CNN) Justice Department inspector general Michael Horowitz said Wednesday that his team did not find any evidence that Trump Tower was under government surveillance, after conducting an exhaustive review of FBI surveillance during the Russia investigation.

Horowitz debunked a conspiracy theory that President Donald Trump invented weeks after taking office in 2017. Horowitz made the comment at a Senate Homeland Security hearing about his report into the early stages of the Russia investigation.

"Did the report find that the FBI engaged in surveillance of Trump Tower?" Sen. Tom Carper, a Delaware Democrat, asked Horowitz.

Horowitz replied, "We did not find evidence of surveillance on Trump Tower."

Trump levied the bombshell accusation in 2017 against former President Barack Obama, claiming that Obama personally ordered his phones to be wiretapped before the 2016 election, and has repeated versions of it since.