A key witness in the special counsel’s investigation was arrested Monday on charges that he transported child pornography.

George Nader, a longtime adviser to the crown prince of the United Arab Emirates, was charged on April 19, 2018 after FBI investigators found 12 videos on one of Nader’s phones showing children between the ages of three and 14 engaging in what the government calls “sexually explicit conduct.”

Nader was initially detained at Washington, D.C.’s Dulles airport on Jan. 17, 2018. The charges were not unsealed until Monday, when Nader was arrested at John F. Kennedy International Airport.

Nader, who is a citizen of both Lebanon and the U.S., was involved in several key meetings at the center of the special counsel’s probe. He helped set up a meeting in the Seychelles in January 2017 between Erik Prince, a Trump supporter who founded the private security firm Blackwater, and Kirill Dmitriev, the head of the Russia Direct Investment Fund.

Nader is mentioned nearly 120 times in the special counsel’s report. He also met numerous times with the special counsel’s office. All but one of the meetings were conducted under a proffer agreement, meaning that he would not be charged with any crimes in exchange for his cooperation.

According to the report, Dmitriev asked Nader to introduce him to Trump transition officials.

Nader also reportedly wired $2.5 million to Elliot Broidy, the embattled Republican party fundraiser, as part of a campaign to oppose Qatar, a geopolitical foe of the UAE.

Nader, 60, pleaded guilty to a similar child porn charge in 1991 in Virginia. He faces a mandatory minimum sentence of 15 years in prison if convicted, prosecutors said.