Esteban Santiago, the alleged gunman who carried out a mass shooting at the Ft. Lauderdale International Airport Friday, specifically traveled to that city to carry out his attack, although it’s not clear why, the FBI said Saturday.

As ABC News reports, Santiago, 26, has been cooperating with investigators as they try to piece together the events that led to Friday’s deadly shooting spree, which left five people dead and six others injured.

#BREAKING Fort Lauderdale airport shooter a passenger on Canadian flight, checked gun in bag,…. https://t.co/advhJeMtrF via @CBS12 pic.twitter.com/1BH4Vwmc7n

— Traditional American (@RightWingAngel) January 6, 2017

FBI Agent George Piro said Santiago, speaking to the media on behalf of the agency, said that Santiago specifically chose Ft. Lauderdale for his shooting spree. It’s not clear, as of this writing, why he chose the city.

“Indications are that he came here to carry out this horrific attack. 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.”

What’s known for certain is that Santiago boarded a Delta Airlines flight in Alaska Thursday night, bound for Ft. Lauderdale. He only checked one piece of luggage: the 9mm semiautomatic handgun, that he legally possessed and transported.

Any legal gun owner in the U.S. can legally transport their weapon, and ammunition, on a passenger plane going from one U.S. city to another. It must be placed in checked baggage – not carry-on – and the gun must be locked and unloaded, and in a hard-sided container.

It’s perfectly legal to transport your gun on an airplane – in your checked luggage. [Image by robert paul van beets/Shutterstock]

After claiming his baggage, investigators say, Santiago went to a bathroom and loaded his gun. When he emerged, he started shooting.

Witness Mark Lea said that, as terrified passengers dropped to the floor, Santaigo went looking for people who were hiding.

“People started kind of screaming and trying to get out of any door they could or hide under the chairs. He just kind of continued coming in, just randomly shooting at people, no rhyme or reason to it.”

Another witness, Bruce Hugon, said a woman next to him who had taken cover tried to stand up, and was immediately shot in the head.

“The guy must have been standing over me at one point. I could smell the gunpowder. I thought I was about to feel a piercing pain or nothing at all because I would have been dead.”

Investigators are still trying to piece together what led the Iraq War veteran to open fire on innocent people. However, the picture that appears to be emerging is one of a troubled former service member who failed to get the help he needed.

Santiago’s mother said that Esteban had been “tremendously affected” by seeing a bomb in Iraq kill two of his friends when he was about 18 or 19 years old. After returning from Iraq, Santiago was later demoted for poor performance. According to his brother, Bryan Santiago, Esteban had told him in August that he was hearing voices.

In November, Esteban Santiago went to an FBI office in Alaska and announced that the government was controlling his mind and forcing him to watch ISIS videos. There, agents questioned the “agitated and disjointed” man and then turned him over to the local cops, who took him to a local hospital for a mental health evaluation.

He was not placed on a no-fly list.

Bryan Santiago questions how a man who admitted to being mentally ill was not given satisfactory treatment and, even moreso, allowed to keep his gun.

“How is it possible that the federal government knows, they hospitalize him for only four days, and then give him his weapon back?”

This is a developing story. More information about alleged Ft. Lauderdale gunman Esteban Santiago will be provided as it becomes available.

[Featured Image by Broward Sheriff’s Office via AP]