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.

President Donald Trump has been testing the boundaries of his authority over US immigration policy since he began his second term of office, but he hinted at taking things to a new level when he said he’d support deporting US citizens to a prison in El Salvador.