The gunman had pleaded guilty to the charges in federal court. He still faces a state murder trial later this year, and Texas prosecutors said they would seek the death penalty.
About 20 migrants, who said they were mainly from Venezuela, arrived on Monday from the same airport in New Mexico where another planeload had taken off Friday.
The state attorney general’s office said the migrants carried documents that specified a Florida government agency and a firm that dropped migrants in Martha’s Vineyard last year.