Even among the brutal ranks of the transnational gang called MS-13, Vladimir Arévalo Chávez stands out as a highly effective manager of murder, prosecutors say.

Known as “Vampiro,” he has been accused of overseeing killings in at least three countries: of migrants in Mexico, rivals in El Salvador and his own compatriots in the United States.

His arrest in February 2023 was a major triumph for American investigators, who only months earlier had accused him and 12 other gang leaders of terrorism, bloodshed and corruption in a wide-ranging federal indictment on Long Island.

But this April, the prosecutors who brought those charges suddenly — and quietly — asked a federal judge to drop them. Citing “national security concerns,” they said they needed to return Mr. Arévalo to El Salvador, his homeland.

The surprising reversal came shortly after a deal the Trump administration struck this year with Nayib Bukele, El Salvador’s president, who agreed to accept more than 200 migrants expelled from the United States into a prison he built for terrorists.

In exchange for helping President Trump carry out his deportation agenda, the United States paid El Salvador millions of dollars, adding an important sweetener at Mr. Bukele’s request: the return of key MS-13 leaders in American custody.