It's been revealed the weapon used to kill five and injure eight more at Fort Lauderdale Airport was handed to the gunman by the FBI.
Esteban Santiago had his gun confiscated when he was referred to psychiatric services weeks before the attack.
It was then returned to him and authorities now say he used the very same firearm to carry out the airport massacre.
Santiago is accused of fatally shooting five people and wounding six others on Friday (local time) at a Fort Lauderdale-Hollywood International Airport baggage claim.
The FBI says Santiago travelled from Alaska to Fort Lauderdale specifically to carry out the shooting.
In November, Santiago had walked into an FBI field office in Alaska saying the US government was controlling his mind and forcing him to watch Islamic State group videos, authorities said.
A law enforcement agent told CBS the 9mm handgun used to kill five people was the same handgun he had in his possession when he went to the office.
"He was a walk-in complaint. This is something that happens at FBI offices around the country every day," FBI agent Marlin Ritzman said.
US District Attorney Karen Loeffler said there is a "difficult standard" in allowing guns to be in possession of mentally ill people, as the law requires them to be "adjudicated mentally ill".