From NYT investigation:

U.S. and Salvadoran officials have said MS-13 leaders are being returned to face justice in El Salvador. But The Times found that American prosecutors have amassed substantial evidence of a corrupt pact between the Salvadoran government and some high-ranking MS-13 leaders, who agreed to drive down violence and bolster Mr. Bukele politically in exchange for cash and perks in jail. Investigators had even begun scrutinizing Mr. Bukele himself.

Some who were part of the federal investigation worry that Mr. Bukele wants the gang leaders back to prevent them from revealing damaging information about his government.