The gunman who shot two people to death at a US shopping mall at the weekend had no criminal record, but little else is known about the 19-year-old and whether he knew his victims before he killed them.
Darion Marcus Aguilar of College Park, Maryland, carried out Saturday's attack with a 12-gauge shotgun at a skateboard shop at the Mall in Columbia in suburban Baltimore before killing himself, police say.
Howard County Police Chief William McMahon says investigators are trying to determine whether Aguilar knew either of the victims, who were both employees of a shop called Zumiez, which sells skateboards, clothing and accessories and is on the mall's upper floor.
Police identified the victims as 21-year-old Brianna Benlolo of College Park, and 25-year-old Tyler Johnson of Mount Airy, Maryland.
McMahon offered no motive for the shooting.
"There are a lot of unanswered questions," McMahon said at a news conference on Sunday.
Aguilar purchased the shotgun legally last month at a store in neighbouring Montgomery County.
It took hours to identify the gunman as he was carrying ammunition and a backpack containing homemade explosives, McMahon said.
Officers searched Aguilar's home Saturday night, recovering more ammunition, computers and documents, police said.
The home is a two-storey wood-frame house in a middle-income neighbourhood called Hollywood. No one answered the door Sunday morning at the house, which had a Christmas wreath on the front door, signs that read "Beware of Dog" and advertised an alarm system.
Aguilar and his mother were renters at the home.
Residents described the neighbourhood as a mix of owners and renters, including some University of Maryland students.
Katie Lawson, director of communications at the University of Maryland, said campus police told her that Aguilar was not and never had been a student there. She said she had no information on the two victims.
Aguilar graduated in 2013 from James Hubert Blake High School in Silver Spring, Maryland, said Dana Tofig, a Montgomery County schools spokesman.
A person who attended the high school with Aguilar told The Associated Press that he was an avid skateboarder.
Tydryn Scott, 19, said she was Aguilar's lab partner in science class and described him as tall, skinny and quiet. She said he was interested in skateboarding and hung out with other skaters.
Scott said she was stung by the news that he was the shooter.
"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 said.
"If any other emotion, he was happy, laughing."
Five other people were hurt in the attack and its aftermath, but only one was hit by gunfire. All were released from hospitals by Saturday evening, police said.
At the time of the shooting, the mall was filled with weekend shoppers and employees.
"There were a lot of people very close to where this happened," Howard County Executive Ken Ulman said.