Since March, the U.S. government has sent Venezuelans and Salvadorans accused of being affiliated with the Tren de Aragua and MS-13 gangs to El Salvador, where Mr. Bukele agreed to hold convicted criminals for the United States, for a fee.

Abrego Garcia, who reportedly fled political persecution from El Salvador and was deported last month by United States Immigration and Customs Enforcement following an "administrative error," remains in the CECOT prison despite court orders requesting the U.S. government "facilitate" bringing him back to the United States.