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, New York.

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 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.

The United States paid El Salvador millions of dollars to help President Donald Trump carry out his deportation agenda, adding an important sweetener at Bukele’s request: the return of key MS-13 leaders in U.S. custody.

Officials from both countries have said the gang leaders are being sent back to face justice.

But the Trump administration has not acknowledged another reason Bukele would want them back: U.S. prosecutors have amassed substantial evidence of a corrupt pact between the Salvadoran government and some high-ranking MS-13 leaders, who they say agreed to drive down violence and bolster Bukele politically in exchange for cash and perks in jail, a New York Times investigation found.

The deal with El Salvador heralded by Trump as a crackdown on crime is actually undermining a long-standing U.S. inquiry into the gang, according to multiple people with knowledge of the initiative. Two major ongoing cases against some of the gang’s highest-ranking leaders could be badly damaged, and other defendants could be less likely to cooperate or testify in court, they said.

The deal has also undercut Trump’s own repeated pledges to dismantle MS-13, a central plank of his tough-on-crime brand. Earlier this year, Attorney General Pam Bondi issued an internal memo calling for the “total elimination” of the gang, which the White House has designated as a foreign terrorist organization.

In his first term, Trump established a group of prosecutors and investigators known as Task Force Vulcan that brought expansive cases against the gang’s leadership. Some who were part of that effort are now alarmed by the deal with Bukele, and worry that he wants the gang leaders back to prevent them from revealing damaging information about his government.

The Times investigation found that U.S. officials have had strong indications for years of the troubling relationship between the Bukele administration and MS-13 and its leaders — and had begun scrutinizing Bukele himself. The findings are based on government documents and interviews with more than 30 people who have knowledge of the Vulcan inquiry or the U.S.-El Salvador relationship, many of whom spoke on the condition of anonymity to describe the ongoing federal investigation.

While he broadly supports Trump’s deportation of gang members, Musto said it was striking to see him host Bukele at the White House to celebrate their deal.

“He was dirty,” Musto said of the Salvadoran president. “He was corrupt. And now he’s sitting next to the president in the Oval Office and he’s got prime access to the leader of the free world.”

Bukele has repeatedly denied there was a pact between his government and gang leaders. Salvadoran officials did not respond to requests for comment.