The Obama administration imposed sanctions on the fund as part of a raft of economic penalties after the Russian government sent military forces into Ukraine in 2014.

The United Arab Emirates, which Washington considers one of its closest Arab allies, co-invested together with Mr. Dmitriev’s fund as part of an effort to build close relations to Russia as well. After Crown Prince Mohammed met with Mr. Putin in 2013 in Moscow on a state visit, two investment arms of the government in Abu Dhabi committed to invest $6 billion in the fund’s Russian projects, eventually paying to build projects like roads, an airport and cancer treatment centers in Russia.

Mr. Dmitriev became a frequent visitor to Abu Dhabi, and Emirati officials came to see him as a key conduit to the Russian government. In a 2015 email, the Emirati ambassador to Moscow at the time described Mr. Dmitriev as a “messenger” to get information directly to Mr. Putin. The email was among a large number hacked from the account of the ambassador to Washington and published online. The now former ambassador to Moscow, Omar Saif Ghobash, did not respond to an email about the leak.

Mr. Nader was first served with search warrants and a grand jury subpoena on Jan. 17, shortly after landing at Washington Dulles International Airport, according to two people familiar with the episode. He had intended to travel on to Mar-a-Lago, Mr. Trump’s Florida estate, to celebrate the president’s first year in office, but the F.B.I. had other plans, questioning him for more than two hours and seizing his electronics.

Since then, Mr. Nader has been questioned numerous times about meetings in New York during the transition, the Seychelles meeting and meetings in the White House with two of Mr. Trump’s senior advisers, Jared Kushner and Stephen K. Bannon, who has since left the administration.

The meeting in the Seychelles also took place against the backdrop of a larger pattern of secretive contacts between the Trump team and both the Russians and the Emiratis. In the weeks after the 2016 presidential election, Crown Prince Mohammed aroused the suspicions of American national security officials when they learned that he had breached protocol by visiting Trump Tower in Manhattan without notifying the Obama administration of his visit to the United States.