The leader of the opposition research firm behind the infamous anti-Trump 'dirty dossier' told Congress last year that the FBI collected intelligence from inside the president's campaign before the 2016 election.

The agency was at the time run by James Comey, the now decidedly anti-Trump former official who has unleashed scathing criticism of Trump in his memoir

Glenn Simpson's testimony is among the disclosures in Wednesday's Senate Judiciary Committee release of transcripts from its investigation into the unverified collection of dirt funded by the Hillary Clinton campaign and the Democratic National Committee.

In August 2017 Simpson told committee members and staff that former British sy Christopher Steele – whom he had hired to probe Trump's potential Russia ties – told him about a meeting in Rome with an FBI agent he knew.

'Essentially what he told me was they [the FBI] had other intelligence about this matter from an internal Trump campaign source,' Simpson recalled.

Fusion GPS boss Glenn Simpson told a Senate committee in 2017 about a conversation he'd had with British ex-spy Christopher Steele

Steele, hired by Simpson's firm to compile the 'dirty dossier' of anti-Trump research, said he was in contact with the FBI in Rome – who told him they had a human intelligence source inside the Trump campaign

Sen. Dianne Feinstein, the Judiciary Committee's top Democrat (right) released Simpson's interview transcript in January

Despite reporting that Steele had misspoken, Simpson's lawyer confirmed to the Senate that he 'stands by his testimony'

He said the Bureau 'believed Chris's information might be credible because they had other intelligence that indicated the same thing, and one of those pieces of intelligence was a human source from inside the Trump organization.'

Simpson moments later said the mole in Trumpworld 'was a voluntary source, someone who was concerned about the same concerns we had. It was someone like us who decided to pick up the phone and report something.'

Simpson's words were first made public in January when California Democratic Sen. Dianne Feinstein released a transcript of Simpson's interview.

Feinstein drew howls from Republicans for unsealing the record without consulting with them first.

President Trump called her 'Sneaky Dianne Feinstein' and blamed her for releasing the testimony 'in such an underhanded and possibly illegal way, totally without authorization.'

James Comey was director of the FBI during the entirety of Trump's presidential campaign, leading the Bureau during the time Simpson says it had a mole inside it

Trump has called the swirl of investigations surrounding his alleged but unproven Russia ties a 'witch hunt'

Trump said in morning tweet that Feinstein may have broken the law when she made the transcript public yesterday, bucking Republican leadership

Simpson's testimony transcript includes this section where he described his conversation with his contractor Christopher Steele about his interactions with the FBI

Simpson said Steele gave him the impression that the FBI's source in Trumpworld was 'voluntary' in nature

A flurry of reporting ensued, with partisans leaking claims to NBC News and other outlets claiming that Simpson had misspoken – that the FBI didn't have a human intelligence source inside the Trump campaign.

Two days later, Committee chairman Chuck Grassley wrote to Simpson's lawyer Joshua Levy for clarification.

Levy responded to the Iowa Republican senator: 'I am writing in response to your letter, dated January 11, 2018, in which you have asked about the August 22, 2017 testimony from our client Glenn Simpson that Christopher Steele in the fall of 2016 said he believed the FBI had another source within the Trump organization/campaign.'

'Mr. Simpson stands by his testimony.'

Most of the material released Wednesday concerns a now-infamous 2016 meeting at Trump Tower between Donald Trump Jr., a British music publicist and a Russian lawyer who the future president's son believed had opposition research about Democratic presidential nominee Hillary Clinton.