Sean Spicer, the White House press secretary, did not respond to an email request to speak with someone in the Trump administration about its thinking regarding the effort. President Trump was a fierce critic of the nuclear accord during the campaign, saying President Barack Obama had negotiated a bad deal. But he has since given no public indication that he intends to follow through on his vow to abrogate it.

The embassy of Luxembourg in Washington also did not respond to a request for comment.

Complicating matters, the lawsuit that resulted in the default judgment did not stem from one of the attacks by Shiite terrorists that specialists generally agree were sponsored by Iran. Instead, it was brought by victims of the Sept. 11, 2001, attacks by Al Qaeda, the Sunni terrorist group.

In 2011, the victims persuaded a federal judge in New York, George B. Daniels, to find that Iran had aided the attacks by providing assistance to Al Qaeda, like facilitating the travel of Qaeda members through its territory. In 2012, he ordered Iran to pay the victims $2 billion in compensatory damages and $5 billion in punitive damages.

That judgment stagnated for years because there was no obvious way to collect it. But then it came to light that the Clearstream system in Luxembourg, which facilitates international exchanges of securities, was holding $1.6 billion in Iranian central bank assets that had been blocked under sanctions.

Last year, lawyers for the Sept. 11 victims persuaded a judge in Luxembourg to place a new freeze on those assets while they sued over whether they could execute the default judgment against those funds, the letter said. Both Clearstream and the Iranian central bank, Markazi, are now trying to get that freeze lifted.

The two lawyers who signed the letter and are leading the effort, Lee S. Wolosky and Michael J. Gottlieb, both partners in the law firm Boies Schiller Flexner, are former Obama administration officials. Mr. Gottlieb was a lawyer in the White House in Mr. Obama’s first term, and Mr. Wolosky served as the special envoy for Guantánamo closure under Secretary of State John F. Kerry, who negotiated the Iran nuclear deal.