Former FBI Director James Comey made a bold assertion about the Hillary Clinton "dirt" tipster's ties to Russia that goes further than what special counsel Robert Mueller said in his report about his investigation into Russian interference in the 2016 election.

In an op-ed on Tuesday, Comey referred to Joseph Mifsud as an "agent" of Russia.

"In April 2016, that adviser talked to a Russian agent in London, learned that the Russians had obtained 'dirt' on Hillary Clinton in the form of thousands of emails and that the Russians could assist the Trump campaign through the anonymous release of information damaging to Clinton," Comey wrote in the Washington Post.

Comey did not name Mifsud, but Mifsud is known as a London-based professor and former Maltese diplomat who has long been suspected of deep ties to Russian intelligence. The original counterintelligence investigation into Trump's campaign began in July 2016 after Papadopoulos repeated what he heard from Mifsud to Australian diplomat Alexander Downer, who informed the U.S. government.

Mueller repeatedly mentions Mifsud in his 448-page report, released last month with redactions, but does not call Mifsud an agent of Russia. The special counsel does say that Mifsud has "connections to Russia" and noted that he "traveled to Moscow in April 2016" and "met with high-level Russian government officials" while he was there before telling Papadopoulos in London about the Clinton "dirt."

Mueller wrote that "Mifsud maintained various Russian contacts while living in London," "offered to introduce Papadopoulos to European leaders and others with contacts to the Russian government," and worked with two Russian nationals, including Russian International Affairs Council member Ivan Timofeev, to arrange a meeting between the Trump campaign and the Russian government that never came to pass.

Mifsud has been an elusive figure who has stayed out of the spotlight over the course of the 22-month Russia investigation, which found no criminal conspiracy between the Trump campaign and the Kremlin, but in recent weeks he has been subject to renewed scrutiny by GOP investigators who believe he may be tied to Western intelligence.

Rep. Devin Nunes, the top Republican on the House Intelligence Committee, says Mifsud likely has links to "U.S., British, and Italian intelligence services" and accused Mueller's team of deciding to "cherry-pick" information from news reports to portray him as a Russian agent, leaving out that he was described as a Western intelligence asset.

In anticipation of Trump's trip to the United Kingdom next month, Nunes provided a list of questions to ask Prime Minister Theresa May, which touches on Mifsud. "Describe any communications or relationship, if any, Joseph Mifsud (potentially also known as Joseph di Gabriele) has had with British intelligence and any information the British government possesses about Mifsud's connection to any other government or intelligence agency," Nunes wrote to Trump last week.

Nunes also said the FBI has "something to hide" after sending two letters to the Central Intelligence Agency, the FBI, the National Security Agency, and the State Department asking for all documents they have on Mifsud, and all but the bureau cooperated.