WASHINGTON — A cooperating witness in the special counsel investigation into Russian election interference has been charged with possession of child pornography, according to court documents unsealed on Monday.

George Nader, a Lebanese-American businessman who acted as an informal adviser to the United Arab Emirates’ powerful crown prince, was arrested on Monday after he landed at Kennedy International Airport in New York, according to a Justice Department news release. The child pornography charges were unsealed after his arrest.

According to the court documents, Mr. Nader was caught early last year with iPhones containing sexually explicit videos of young boys. The phones were discovered after he was stopped by federal agents at an airport and questioned about his efforts to arrange meetings between intermediaries from several foreign governments and Trump transition officials in the weeks after the November 2016 election. Mr. Nader had traveled to the United States to attend a party at Mar-a-Lago celebrating President Trump’s first year in office.

Mr. Nader received partial immunity in exchange for his testimony, and he ended up spending hours before a grand jury being questioned by prosecutors working for the special counsel, Robert S. Mueller III.