The suspect in the Fort Lauderdale Airport shooting was reportedly known to the FBI

The man authorities believe shot 13 people, five of them fatally, at Fort Lauderdale International Airport on Friday was known to the FBI prior to carrying out the massacre, CBS News reported.

Law enforcement sources told CBS that the suspected assailant, 26-year-old Esteban Santiago, walked into an FBI office in Anchorage, Alaska, in November 2016 claiming voices in his head were telling him to join ISIS.

Bryan Santiago, the suspect's brother, told The Associated Press that Esteban had been receiving psychological treatment in Alaska, where he had been living since 2014. He said he did not know what his brother was being treated for.

Santiago, who was born in New Jersey but moved to Puerto Rico with his family when he was 2 years old, served in Puerto Rico's National Guard and was deployed to Iraq in 2010, Puerto Rico's National Guard spokesman Maj. Paul Dahlen told The Associated Press.

Santiago received a "general discharge" from the National Guard in August for "unsatisfactory performance," a lieutenant with the Alaska National Guard told CBS.

His aunt, who lives in New Jersey, told reporters that Santiago had started acting strangely when he returned from Iraq, and about a month ago "lost his mind."

"He said he saw things," his aunt, Maria Luisa Ruiz, told reporters outside her home in Union City, New Jersey. She mentioned that he had just become a father in September.

Santiago was charged with fourth-degree assault about one year ago in Anchorage in an incident related to domestic violence, police told the Daily Beast.

Santiago flew Delta from Anchorage, Alaska to Fort Lauderdale via Minneapolis-St. Paul on Thursday night, NBC reported. He arrived in Fort Lauderdale late Friday morning, which is when he apparently he used a gun he had checked into his luggage to carry out the massacre in the baggage claim area of Terminal 2.

Santiago is being interviewed by a team of FBI agents and homicide detectives, Broward County Sheriff Scott Israel said in a press conference Friday.

"At this point it looks like he acted alone," Israel said of the suspect. He noted that "it's too early to say either way" whether this was an act of terrorism.

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