Investigators found a journal belonging to Darion Marcus Aguilar, 19, but they would only say that it "expressed general unhappiness." The contents, however, were enough for an officer looking into the disappearance of Aguilar on the day of the shooting to worry about the teen's safety.
Police said Aguilar took a taxi to the Mall in Columbia in suburban Baltimore 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 who graduated from high school less than a year ago and had no previous run-ins with law enforcement.
Aguilar, who had concealed the shotgun in a bag, fired six to nine times. 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.
The other employee, Tyler Johnson, did not know Aguilar and did not socialize with Benlolo outside of work, a relative said.
Zumiez chief executive Rick Brooks said in a statement that when the mall reopens, 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.
The Prince George's County Police Department said it received a missing persons report for Aguilar at about 1:40 p.m. Saturday, more than two hours after the mall shooting. Officers went to Aguilar's home to speak with his mother about 5 p.m. and saw Aguilar's journal. The portion the officer read made him concerned for Aguilar's safety, the department said.
Police began tracking Aguilar's phone and soon discovered it was at the mall.
Howard County Police Chief William McMahon said there has been speculation about a romantic relationship between the gunman and Benlolo, but investigators have not been able to establish that.
Aguilar purchased the 12-gauge shotgun legally last month at a store in neighboring Montgomery County.
At his home where he lived with his mother, 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.
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 Zumeiz. All were released from hospitals hours later.