McMahon said police have seen nothing in Aguilar's background that hinted at violence. According to Maryland electronic court records, he had no criminal record. Aguilar lived near Benlolo, but McMahon said he had no indication the two knew each other.
Aguilar was quiet but outgoing, according to friends and former classmates at James Hubert Blake High School in Silver Spring.
"I never ever heard him curse, raise his voice, none of that," said one classmate, Noah Sturdivant. Aguilar had a distinctive fashion sense that Sturdivant described as a mix of graphic art and skateboard culture.
Blake High will have counselors on hand to speak to students today, Principal Christopher S. Berry wrote in a statement posted on the school's website. "It is a sad time of reflection for our wider community," he wrote.
Cropper said his mother never saw a gun or ammunition in the family's College Park home. She was not aware that in December he had gone to Montgomery County to buy the shotgun that police said he used Saturday.
"His mother right now is struggling for a reason to live," Cropper said. "It's become an epidemic right now. Every week we're reading about someone shooting in a mall. … What out there in our world is angering these children that makes them do something like this?"
McMahon said he believed Aguilar purchased the shotgun legally.
Along with the gun, he was found with a large amount of ammunition and a bag containing two crude devices made of "flash powder and household items." Police rendered the devices harmless, McMahon said, and no other booby traps or explosives were found in the mall after 20 canine teams searched the complex.
Five people were treated at an area hospital and released Saturday. A woman was shot in the foot, and four others were hurt during the chaos after the shooting. On Sunday, McMahon clarified that the woman was shot on the mall's second level, where Zumiez is, not on the first level, as first reported.
Benlolo grew up in Florida and Colorado before moving to Maryland in 2010, according to her Facebook page. Johnson's Facebook page said he began working at Zumiez, which sells skateboard apparel and gear, in November. Both have family in Mount Airy.
At Sunday evening's news briefing, Ulman praised emergency responders and said the "outpouring of compassion" in the wider community was "really heartening."
But standing beside him, his police chief predicted that Saturday's shooting would leave a lasting mark.
"Nothing is going to go back to normal," McMahon said. "This mall is going to be different because of what happened."
Baltimore Sun Media Group reporters Yvonne Wenger, Erica L. Green, Colin Campbell and Natalie Sherman contributed to this article.
Two memorial areas will be held when the mall opens at 1 p.m. Monday. One will be in front of the entrance near Sears; the other will take place in the mall's center court.