Last summer, the FBI got a secret court order to monitor the communications of Carter Page, an adviser to then-candidate Donald Trump, The Washington Post reported.
The FBI had convinced a Foreign Intelligence Surveillance Court judge that there was probable cause to believe Page was acting as an agent of Russia, according to the Post's sources.
Whether the Trump campaign coordinated with the Russian government to swing the election is a subject of multiple congressional investigations.
Page has not been charged with any crimes, and it is not clear if the FBI will seek any charges any related to contacts with Russia. Page denied any wrongdoing.
Page worked for Merrill Lynch in Moscow a decade ago and once invested in Gazprom, a Russian energy company. He told Comey he has since sold his shares in the company.
The application for surveillance said Page met with a Russia spy in New York in 2013, among other contacts that were not disclosed.
Page also spoke in Moscow in July, delivering a speech that criticized the U.S.'s Russia policies. He said in September he met Russian Deputy Prime Minister Arkady Dvorkovich during the same trip.
Page is the only American to have had his communications directly targeted with a FISA warrant as part of the Russia investigation, officials said.
Three years before he joined the Trump campaign, the FBI learned that people suspected of being Russian spies wanted to use Page as a source of information.
Victor Podobnyy, who was later charged with acting as an unregistered agent of a foreign government, was recorded saying that a man officials identified as Page spoke to him " flies to Moscow more often than I do" and that "It's obvious that he wants to earn lots of money." Podobnyy later said he would promise Page a favor, get information from him "and tell him to go f--- himself," according to court documents.
A federal court complaint alleges Page gave Podobnyy documents about the energy business and shared his outlook on the industry. He has since said the information he gave was "publicly available research documents."