Darion Marcus Aguilar of College Park, Md., 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 said.
Howard County Police Chief William McMahon said 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.
Police identified the victims as 21-year-old Brianna Benlolo of College Park, Md., and 25-year-old Tyler Johnson of Mount Airy, Md. McMahon offered no motive for the shooting.
"There are a lot of unanswered questions," McMahon said at a news conference. Aguilar purchased the shotgun legally last month at a store in neighboring Montgomery County.
It took hours to identify the gunman since 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-story wood-frame house in a middle-income neighborhood called Hollywood, just off U.S. Route 1 and near the Capital Beltway. 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 warnings about an alarm system.
Aguilar and his mother were renters at the home. Sirkka Singleton, who owns the property with her husband and lives a block away, said they use a property manager to find tenants and they have never met the Aguilars. She declined to say who the property manager was.
Residents described the neighborhood as a mix of owners and renters, including some University of Maryland students.
Katie Lawson, director of communications at the university, said campus police told her that Aguilar has 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, 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.
She 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."
A man who answered the phone at Johnson's residence in Mount Airy, northwest of Baltimore, said the family had no comment. But the victim's aunt told a local television station that she did not believe her nephew knew Aguilar.
Sydney Petty, in a statement to WBAL-TV, also said she did not believe her nephew had a relationship with Benlolo outside work.
"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.
She said her nephew also worked at a drug rehabilitation center in Mount Airy, for which she served on the board.
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 busy with weekend shoppers and employees.
"There were a lot of people very close to where this happened," Howard County Executive Ken Ulman said.
Police searched the mall with dogs overnight, and stores were to remain closed through Tuesday.
Benlolo's grandfather, John Feins, said in a telephone interview from Florida that his granddaughter had a 2-year-old son and that the job at Zumiez was her first since she went back to work after her son's birth.
"She was all excited because she was the manager there," he said.
He described his daughter's family as a military family that had moved frequently and had been in Colorado before moving to Maryland about two years ago. He said his granddaughter was on good terms with her son's father, and they shared custody.
"I mean what can you say?" he said. "You go to work and make a dollar and you got some idiot coming in and blowing people away."
Associated Press writers Jessica Gresko and Martin Di Caro in Washington contributed to this report. Eric Tucker contributed from Columbia, Md., and Kasey Jones reported from Baltimore.