Authorities trying to piece together why a Maryland teenager gunned down two people before killing himself in a mall over the weekend are analyzing a journal belonging to the shooter where he “expressed general unhappiness” with his life.

The journal was found when the Prince George's County Police Department received a missing persons report for 19-year-old Darion Marcus Aguilar at about 1:40 p.m. Saturday, more than two hours after the shooting at the Mall of Columbia near Baltimore.

Officers went to Aguilar's home to speak with his mother at about 5 p.m. and saw the journal. The portion an officer read made him concerned for Aguilar's safety, the department said. Investigators only said the contents of the journal "expressed general unhappiness."

Police began tracking Aguilar's phone and soon discovered it was at the mall, and he was later identified as the shooter.

Authorities say Aguilar took a taxi to the mall on Saturday morning and entered the building near Zumiez, a shop that sells skateboarding gear. He went downstairs to a food court directly below the store, then returned less than an hour later, dumped the backpack in a dressing room and started shooting.

Shoppers fled in a panic or barricaded themselves behind closed doors. When police arrived, they found three people dead -- two store employees and Aguilar.

The shooting baffled investigators and acquaintances of Aguilar, a quiet, skinny teenager described as a skateboarding enthusiast who graduated from high school less than a year ago and had no previous run-ins with law enforcement.

Aguilar showed up at the shopping center armed with a shotgun, plenty of ammunition and a backpack filled with crude homemade explosives, according to police. He purchased the 12-gauge shotgun legally last month at a store in neighboring Montgomery County, and fired it six to nine times inside the mall.

One victim, Brianna Benlolo, a 21-year-old single mother, lived half a mile away from Aguilar in the same College Park neighborhood, but police said they were still trying to determine what, if any, relationship they had.

Howard County Police Chief William McMahon said there has been speculation about a romantic relationship between Aguilar and Benlolo, but investigators have not been able to establish that.

At Aguilar’s home, officers also recovered more ammunition, computers and documents, police said. No one answered the door there Sunday.

A half-mile away, a roommate who answered the door at Benlolo's home confirmed that she lived there but declined to comment further. Two police officers went into the home after he spoke briefly to a couple of reporters.

Residents described the neighborhood as a mix of owners and renters, including some University of Maryland students.

The other employee who was killed, Tyler Johnson, did not know Aguilar and did not socialize with Benlolo outside of work, a relative said.

A man who answered the phone at Johnson's residence in Mount Airy, northwest of Baltimore, said the family had no comment. The victim's aunt told a local television station she did not believe her nephew knew Aguilar.

Sydney Petty, in a statement to WBAL-TV, said she did not believe her nephew had a relationship with Benlolo.

"Tyler didn't have anything beyond a working relationship with this girl, and he would have mentioned it if he did, and we're just as confused as anybody," Petty said.

Five other people were hurt in the attack, but only one was hit by gunfire -- a woman who was hit in the foot upstairs near Zumiez. All were released from hospitals hours later.

Zumiez chief executive Rick Brooks said in a statement that when the mall reopens at 1 p.m. local time Monday, there will be memory books to sign and visitors will be invited to float flowers in the mall's fountain in memory of Benlolo and Johnson.

"Counselors have met with the store team," he said Sunday. "The emotions are very raw and real -- and as co-workers and friends, we are pulling together."

Aguilar was accepted last February to Montgomery College, a community college in the Washington suburbs, but school spokesman Marcus Rosano said he never registered or attended.

Tydryn Scott, 19, said she was Aguilar's lab partner in science class at James Hubert Blake High School and said he hung out with other skaters. She said she was stung by the news.

"It was really hurtful, like, wow -- someone that I know, someone that I've been in the presence of more than short amounts of time. I've seen this guy in action before. Never upset, never sad, just quiet, just chill," Scott told The Associated Press. "If any other emotion, he was happy, laughing."

Aguilar graduated in 2013.

Rep. Mike McCaul, R-Texas, the chairman of the House Homeland Security Committee, said on CBS’ “Face the Nation” Sunday that the shooting represented the “vulnerability of shopping malls.”

“You can't have security lockdown at our malls. But things like canines, heavier canine presence, very good at detecting explosives. I know this individual had a backpack of explosives” he said. "But the fact is… it's very difficult to stop a lone gunman who may have mental issues that wants to kill people. And you can only do so much to stop that.”

The Associated Press contributed to this report.