The man arrested after five people were shot dead at a Florida airport yesterday previously entered FBI offices speaking about mind control, and appears to have been working alone.
Esteban Santiago, 26, opened fire at Fort Lauderdale airport after arriving on a flight from Alaska with a checked gun in his bag, authorities said.
Five people died in the attack and eight were wounded.
He has been charged with carrying out an act of violence at an international airport resulting in death, which carries a maximum punishment of execution, and also faces lesser weapons charges.
Mr Santiago, who is in custody, told agents he had planned the attack and bought a one-way ticket to Fort Lauderdale, according to court papers.
Authorities said they did not know why he chose this target and terrorism had not been ruled out.
An official list of the victims has not been released but families and friends have confirmed some of the identities.
Olga Woltering was a Georgia resident originally from Ipswich in eastern England.
A devout Catholic in her 80s, she was named as a victim by her Atlanta church, the Catholic Church of the Transfiguration. It described her as a "joyful, loving, caring and committed" person.
Ms Woltering, from Marietta near Atlanta, was in Florida on her way to join a cruise to celebrate her husband's 90th birthday. He was unharmed in the shooting.
Terry Andres, 63, from Virginia Beach, Virginia, was a volunteer fireman. He and his wife had flown to Fort Lauderdale for a Caribbean cruise. He was named as a victim by friend Jessica Winbauer.
Santiago previously spoke incoherently of mind control, but had handgun returned to him - FBI
The FBI and Anchorage police said Mr Santiago walked into an FBI office in Alaska in November, agitated and incoherent.
He was carrying a loaded magazine but had left his handgun in his car, with his newborn child.
He was given a mental-health assessment and later discharged.
FBI special agent Marlin Ritzman defended not putting Mr Santiago on a no-fly list after the incident.
"I want to be clear - during our initial investigation we found no ties to terrorism," Mr Ritzman said.
"He broke no laws when he came into our office making disjointed comments about mind control."
During the later mental health evaluation, he told the FBI he was hearing voices and believed he was being controlled by a US intelligence agency.
But the authorities found no wrongdoing, and the gun was returned in December.
The FBI said despite Santiago seeming to have travelled to Fort Lauderdale specifically to carry out the attack, there was no indication had been working with anyone else in planning it.
FBI agent George Piro said Mr Santiago had been questioned at length overnight.
There was no sign of any altercation on the flight or at the airport before the attack began, Mr Piro said.
Federal charges have been filed against Mr Santiago and they should be outlined in detail later on Saturday.
Mr Piro said: "We continue to look at all avenues and all motives for this horrific attack. We are continuing to look at the terrorism angle in regards to the potential motivation."
Gunman's military history, poor mental health
Mr Santiago is a former member of the Puerto Rico and Alaska National Guard, according to the Pentagon.
He served in Iraq from April 2010 to February 2011, and ended his service in August 2016.
US media reported that he had received a general discharge from the Alaska National Guard for unsatisfactory performance.
His brother said he had been receiving psychological treatment recently.
His aunt told a local newspaper he had "lost his mind" while serving in Iraq.
He reportedly told police after the attack that the government was controlling his mind and made him watch jihadist videos.
Fort Lauderdale Airport reopened on Saturday but Terminal 2, where the shootings happened, remained closed.
Airport officials say they still have 20,000 items of luggage to return to their owners.
A tweet from the airport account said this was a "complex and time-consuming process". When the gunman opened fire, many people rushed out on to the tarmac.
A White House spokesman said President Barack Obama had expressed his condolences to the relatives of the victims.
In a tweet, President-elect Donald Trump sent his "thoughts and prayers".
Flying with firearms is legal in the US as long as the guns are kept in a locked, hard-sided container as checked baggage only, under rules of the Transport Security Administration (TSA).
Ammunition is also allowed only in checked luggage.
The attack was the latest in a series of mass shootings in the US in recent years, carried out by people who said they were inspired by jihadist groups, were loners or mentally disturbed, who had easy access to weapons under US gun laws.
Last year, in the worst shooting in recent US history, a man apparently inspired by Islamic State killed 49 people at a gay nightclub in Orlando, Florida.