The Senate Intelligence Committee is considering seeking testimony from the former British spy who alleged in a dossier that the Trump campaign engaged in a "well-developed conspiracy of cooperation" with the Russian government as Russia sought to help Trump win the election, according to two sources familiar with the matter.

Christopher Steele is a former officer in Britain’s Secret Intelligence Service, known as MI-6. He began gathering unfavorable information on Donald Trump last year on behalf of anti-Trump Republicans, and then later worked on behalf of Democrats. His 35-page dossier, a version of which has been posted online, is an amalgam of salacious and explosive claims about Trump, none of which have been publicly proven. But the FBI considered Steele credible enough that it was willing to pay him in October to continue looking into Trump’s Russian ties, NBC News has reported. The arrangement fell through after Steele became frustrated with the FBI.

The dossier cites a number of Russian sources who allege that Russian intelligence agencies have been cultivating Trump for years, have compromising material on him, and colluded with his aides in a dirty tricks campaign to leak negative information hacked from Democrats. Trump has denied all that, calling it "fake news." FBI director James Comey briefed Trump on the dossier before he became president.

Steele is in virtual hiding in Europe, so it’s unclear whether he would be willing to come to Washington. The Independent newspaper of London reported that the Senate Intelligence Committee was considering interviewing him somewhere else.

The FBI and Congressional intelligence committees are investigating whether any Americans, including Trump associates, helped Russia in a campaign of hacking, leaking and fake news that U.S. intelligence agencies say was designed to sow chaos, harm Clinton, and help Trump. Trump aides deny that they colluded with the Russians.

Steele provided crucial help in the FBI's investigation of corruption in FIFA, the international soccer body. He is also known to the CIA, and former officers told NBC News that he was considered a reputable and competent intelligence officer.