A former British spy has reportedly given FBI agents the names of his sources for an explosive 35-page dossier linking President Donald Trump to Russian efforts to interfere with the election.

Former MI6 agent Christopher Steele hasn’t yet spoken to congressional investigators about his findings, but he has met with FBI agents to discuss how he had learned about Trump’s alleged activities involving Russia, reported ABC News.

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Steele was hired by the opposition research firm Fusion GPS to look into Trump’s background after wealthy Republicans paid the Washington, D.C.-based business about $1 million, and then later worked for Democrats who sought additional damaging information.

He was so troubled by his findings that he continued the investigation at his own expense after the election.

Fusion GPS founder Glenn Simpson, a former Wall Street Journal reporter who specialized in money laundering and Russian organized crime, was scheduled to appear in a closed session with congressional investigators to discuss the dossier.

A recent U.S. court ruling could compel Steele to testify before lawmakers.

Russian tech mogul Aleksej Gubarev, who was named in the dossier, has sued Buzzfeed — which published the unverified dossier shortly before Inauguration Day — for libel, and his attorneys want to question Steele under oath about his findings.

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A U.S. District Court judge in Florida recently ruled that Gubarev’s U.S.-based attorneys could seek British approval to force Steele’s testimony.

Attorneys for Fusion GPS have argued that their client relationships are confidential.

Watch this video report for information about Steele’s meetings with the FBI: