While investigators try to figure out the motive of an Iraq war veteran accused of killing five travellers and wounding six others at a busy international airport in Florida, the suspected gunman was charged and could face the death penalty if convicted.
Esteban Santiago, 26, was charged with an act of violence at an international airport resulting in death - which carries a maximum punishment of execution - and weapons charges.
Mr Santiago told investigators that he planned the attack, buying a one-way ticket to the Fort Lauderdale airport, a federal complaint said.
Authorities don't know why he chose his target and have not ruled out terrorism.
"Today's charges represent the gravity of the situation and reflect the commitment of federal, state and local law enforcement personnel to continually protect the community and prosecute those who target our residents and visitors," US Attorney Wifredo Ferrer said.
Authorities said during a news conference that they had interviewed roughly 175 people, including a lengthy interrogation with a cooperative Mr Santiago, who is a former National Guard soldier from Alaska.
Flights had resumed at the Fort Lauderdale airport after the bloodshed, though the terminal where the shooting happened remained closed.
FBI Agent George Piro said Santiago spoke to investigators for several hours after he opened fire with a 9mm semi-automatic handgun that he appears to have legally checked on a flight from Alaska.
"Indications are that he came here to carry out this horrific attack," Mr Piro said.
"We have not identified any triggers that would have caused this attack. We're pursuing all angles on what prompted him to carry out this horrific attack."
Investigators are combing through social media and other information to determine Santiago's motive, and it's too early to say whether terrorism played a role, Mr Piro said.
In November, Mr 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.
"He was a walk-in complaint. This is something that happens at FBI offices around the country every day," FBI agent Marlin Ritzman said.
Mr Santiago had a loaded magazine on him, but had left a gun in his vehicle, along with his newborn child, authorities said.
Officers seized the weapon and local officers took him to get a mental health evaluation. His girlfriend picked up the child.
On December 8, the gun was returned to Mr Santiago. Authorities wouldn't say if it was the same gun used in the airport attack.
US Attorney Karen Loeffler said Mr Santiago would have been able to legally possess a gun because he had not been judged mentally ill, which is a higher standard than having an evaluation.
Mr Santiago had not been placed on the US no-fly list and appears to have acted alone, authorities said.
The attack sent panicked witnesses running out of the terminal and spilling onto the tarmac, baggage in hand.
Others hid in bathroom stalls or crouched behind cars or anything else they could find as police and paramedics rushed in to help the wounded and establish whether there were any other gunmen.
Mr Santiago had been discharged from the National Guard last year after being demoted for unsatisfactory performance. Bryan Santiago said Saturday that his brother had requested psychological help but received little assistance. Esteban Santiago said in August that he was hearing voices.
Mr Santiago, who is in federal custody with no bail, will face federal charges and is expected to appear in court on Monday, Piro said.