The United States’ former National Security Advisor Michael Flynn on Friday pleaded guilty to lying to the Federal Bureau of Investigation about his Russian contacts, and said he would cooperate with their inquiry into Russia’s attempts to influence the 2016 election.

Flynn has struck a plea bargain deal, Reuters reported. In a Washington court, he admitted to lying when asked about his conversations with the former Russian Ambassador Sergei Kislyak before Donald Trump took office as the US president. The developments in the inquiry are also likely to raise questions about Trump’s son-in-law Jared Kushner and his ties with Russia.

Donald Trump has not yet commented on the developments in the investigation. Now that Flynn has cut a deal, special counsel Robert Mueller is likely to get a behind-the-scenes look at the Trump campaign and the early weeks of his administration.

Flynn faces a possible sentence of up to six months in prison in his guilty plea to the false statements charge.

Flynn, who had ties with Russia even before the election campaign began, was an early and important adviser to Trump and shaped the president’s populist “America first” platform. He was also the person in the transition team in charge of dealing with Kremlin.

Flynn resigned in February, barely a month after the Trump administration took over, after it came to light that that he had discussed the country’s sanctions on Russia with the then Russian envoy Sergei Kislyak. He is also being investigated by the Justice Department for failing to properly disclose his foreign business dealings.