They arrested him because they claimed he is one of the gang members. He wasn't, and the US government said it was a mistake. He lived in the US lawfully with a legal work permit and his wife is a US citizen and pregnant. There is not one court paper filed that said he wasn't lawfully in the US. This is what the Court stated: "To this day, the Government has cited no basis in law for Abrego Garcia’s warrantless arrest, his removal to El Salvador, or his confinement in a Salvadoran prison. Nor could it."