Esteban Santiago is taken from the Broward County main jail as he is transported to the federal courthouse in Fort Lauderdale, Fla., on Monday, Jan. 9, 2017. Santiago is accused of fatally shooting several people at a crowded Florida airport baggage claim and faces airport violence and firearms charges that could mean the death penalty if he's convicted. (Amy Beth Bennett/South Florida Sun-Sentinel via AP)
Esteban Santiago is taken from the Broward County main jail as he is transported to the federal courthouse in Fort Lauderdale, Fla., on Monday, Jan. 9, 2017. Santiago is accused of fatally shooting several people at a crowded Florida airport baggage claim and faces airport violence and firearms charges that could mean the death penalty if he's convicted. (Amy Beth Bennett/South Florida Sun-Sentinel via AP)

Anchorage, Alaska — Esteban Santiago stood alone in the cold one day last month outside Mom & Pop’s liquor store in Anchorage. He was waving his arms and having a terrible argument in the parking lot.

“He’d just be talking to himself … screaming as if he was having a battle with himself,” said Naomi Harden, a clerk at the store, situated across the street from the motel where Santiago lived.

Last week, Santiago got off a one-way flight from Anchorage at the airport in Fort Lauderdale, Fla., arriving with a single piece of checked luggage: a Walther 9 mm semi-automatic handgun in a case. He took the gun out in a bathroom, loaded it and opened fire in the baggage claim area, killing five people and wounding six, police say.

Investigators are trying to establish the motive for the attack.

The 26-year-old Iraq War veteran was usually a quiet and solitary figure, those who knew and worked with him said. But over the past year, he grew increasingly violent, interviews and records show.

Santiago grew up in Penuelas, Puerto Rico, a once-thriving middle-class town of 20,000 where people worked in petrochemical plants that have since closed. Those who knew him described an intelligent and patriotic boy who joined the Puerto Rico Army National Guard in 2007.

Neighbors would salute him as he walked down his neighborhood’s narrow streets.

Still, he tended to feel more comfortable alone, his brother, Bryan Santiago, said in Spanish, standing outside the family’s home in Penuelas.

Santiago served about a year in Iraq in 2010 as part of an engineering battalion, clearing roadside bombs and repairing bridges. While there, his family said, he witnessed a bomb explode near two friends.

In 2014, Santiago and his brother moved to Anchorage for a new start. Esteban joined the Alaska National Guard.