Special Counsel Robert Mueller has recommended a sentence up to six months in prison for former campaign aide of President Donald Trump, George Papadopoulos, who is charged with lying to federal agents in investigations whether Russia interfered in the 2016 U.S. presidential election.

“The defendant knew the questions he was asked by the FBI were important, and he knew his answers were false at the time he gave them. His lies negatively affected the FBI’s Russia investigation, and prevented the FBI from effectively identifying and confronting witnesses in a timely fashion,” the court document states. Mueller said the government believed a sentence of up to six months in prison was “appropriate and warranted” along with a fine of $9,500.

Papadopoulos lied to the FBI in January 2017 about his foreign contact in London with Joseph Mifsud, identified in the filing as “the Professor.” Mifsud had told Papadopoulos the Russians had “dirt” on Hillary Clinton. Federal authorities then found Mifsud in D.C. two weeks later, but allowed him to leave the country. “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,” prosecutors wrote. “The defendant’s false statements were intended to harm the investigation, and did so.”

In October last year, the former Trump staffer pleaded guilty of lying to investigators. He reached a plea agreement with Mueller’s team and is set to be sentenced in September. Two months after he cut his plea deal, an additional meeting in December, where investigators wanted to ask Papadopoulos more questions, was canceled because he and his then-fiancée, Simona Mangiante, began giving interviews to the media. Now, Mangiante is urging her husband to break off the plea deal and stop cooperating with Mueller.

“I trusted the institutions until they proved me wrong,” Mangiante told MSNBC on Thursday, adding that she had learned of “exculpatory evidences that fully justify him to drop off his plea agreement.”

Another defendant, Alex van der Zwaan, also pleaded guilty to lying to investigators. He received 30 days in prison.