WASHINGTON (AP) — A former Trump campaign adviser should spend at least some time in prison for lying to the FBI during the Russia probe, prosecutors working for special counsel Robert Mueller said in a court filing Friday that also revealed several new details about the early days of the investigation.

The prosecutors disclosed that George Papadopoulos, who served as a foreign policy adviser to President Donald Trump’s campaign during the 2016 presidential race, caused irreparable damage to the investigation because he lied repeatedly during a January 2017 interview.

Those lies, they said, resulted in the FBI missing an opportunity to properly question a professor Papadopoulos was in contact with during the campaign who told him that the Russians possessed “dirt” on Hillary Clinton in the form of emails.

The filing by the special counsel’s office strongly suggests the FBI had contact with Professor Joseph Mifsud while he was in the U.S. during the early part of the investigation into Russian election interference and possible coordination with Trump associates.

According to prosecutors, the FBI “located” the professor in Washington about two weeks after Papadopoulos’ interview and Papadopoulos’ lies “substantially hindered investigators’ ability to effectively question” him. But it doesn’t specifically relate any details of an interview with the professor as it recounts what prosecutors say was a missed opportunity caused by Papadopoulos.

“The defendant’s lies undermined investigators’ ability to challenge the Professor or potentially detain or arrest him while he was still in the United States,” Mueller’s team wrote, noting that the professor left the U.S. in February 2017 and has not returned since.

Prosecutors note that investigators also missed an opportunity to interview others about the professor’s comments or anyone else at that time who might have known about Russian efforts to obtain derogatory information on Clinton during the campaign.

“Had the defendant told the FBI the truth when he was interviewed in January 2017, the FBI could have quickly taken numerous investigative steps to help determine, for example, how and where the Professor obtained the information, why the Professor provided the information to the defendant, and what the defendant did with the information after receiving it,” according to the court filing.

Prosecutors also detail a series of difficult interviews with Papadopoulos after he was arrested in July 2017, saying he didn’t provide “substantial assistance” to the investigation. Papadopoulos later pleaded guilty to lying to the FBI as part of a plea deal.

The filing recommends that Papadopoulos spend at least some time incarcerated and pay a nearly $10,000 fine. His recommended sentence under federal guidelines is zero to six months, but prosecutors note another defendant in the case spent 30 days in jail for lying to the FBI.

Papadopoulos has played a central role in the Russia investigation since its beginning as an FBI counterintelligence probe in July 2016. In fact, information the U.S. government received about Papadopoulos was what triggered the counterintelligence investigation in the first place. That probe was later take over by Mueller.

Papadopoulos was also the first Trump campaign adviser to plead guilty in Mueller’s investigation.

Since then, Mueller has returned two sweeping indictments that detail a multi-faceted Russian campaign to undermine the U.S. presidential election in an attempt to hurt Clinton’s candidacy and help Trump.

Thirteen Russian nationals and three companies are charged with participating in a conspiracy to sow discord in the U.S. political system primarily by manipulating social media platforms.

In addition, Mueller brought an indictment last month against 12 Russian intelligence operatives, accusing them of hacking into the computer systems of Clinton’s presidential campaign and the Democratic Party and then releasing tens of thousands of private emails through WikiLeaks.

According to that indictment, by April 2016, the Russian intelligence operatives had already stolen emails from several Democratic groups including the Clinton campaign and were beginning to plan how they were going to release the documents. That same month, according to court papers, Mifsud told Papadopoulos that he had met with senior Russian government officials in Moscow and had learned that they had “dirt” on Clinton in the form of “thousands of emails.”