In one case, a petition filed by immigrant detainees said the government’s supposed evidence for his gang membership included emojis he used online.

A spokeswoman for the presidency of El Salvador, Wendy Ramos, did not respond to requests for information about Senator Van Hollen’s meeting with Abrego Garcia, including whether he had been returned to the prison where American officials have said he is being held, the Terrorism Confinement Center, or CECOT.