George Nader, a man who served as a key witness in special counsel Robert Mueller’s probe into Russian interference in the 2016 presidential election, was charged with transporting child pornography and arrested in New York on Monday.

Federal prosecutors allege Nader, 60, traveled with illegal material on his cell phone while flying from Dubai to Washington D.C. in early 2018. He allegedly had visual depictions of “minors engaged in sexually explicit conduct” at the time, and was arrested Monday upon arrival at John F. Kennedy International Airport.

He faces between 15 to 40 years in prison if convicted. He previously pleaded guilty to the same charge in 1991, serving about six months in prison.

Nader, a Lebanese-American businessman, served as an informal advisor to the United Arab Emirates’ crown prince and had links to several aides to President Donald Trump. Within Mueller’s report, Nader is cited multiple times and was depicted as a liaison between high-level people in the Middle East and Russia who hoped to make inroads with the Trump camp.

“Nader developed contacts with both U.S. presidential campaigns during the 2016 election,” Mueller’s report reads at one point. He sat for multiple voluntary interviews in exchange for some legal protection in the special counsel’s inquiry.

Before the 2016 election, Nader reportedly met with Donald Trump Jr. and Erik Prince, the founder of the private security firm Blackwater and then an informal advisor to Trump, telling the pair that the UAE and Saudi Arabia could help Trump get elected, The New York Times reported.

At one point after the election, Nader also helped arrange a secret meeting in the Seychelles between the UAE’s Crown Prince Mohammed bin Zayed, Kirill Dmitriev, the head of Russia’s state-controlled wealth fund, and Prince. Nader also met regularly with Steve Bannon, who was a senior advisor to the president at the time, and Jared Kushner, the president’s son-in-law.

Nader was stopped in 2018 at Dulles International Airport by federal agents who served him a subpoena and asked to speak with him as part of Mueller’s investigation. His three iPhones were seized at the time, where investigators then discovered the child pornography.

He was charged under seal in April 2018, but had left the country by that time, according to The Washington Post.