The gunman who shot and killed two people at a US mall was a 19-year-old man who lived with his mother in the Maryland city of College Park and arrived at the shopping centre in a taxi about an hour before opening fire.
But a day after the shootings, police could provide no immediate insight into why Darion Marcus Aguilar killed a young man and a young woman at the mall in Columbia, Maryland, west of Baltimore, before apparently killing himself.
Police have yet to find any ties between Aguilar and the two people he killed, employees at a skate shop at the mall, Howard County Police Chief Bill McMahon said.
"We do know that one of our victims also lived in College Park," McMahon said. "We don't know if there's any connection there or whether that's merely coincidental."
Aguilar fired six to eight shots from a 12-gauge shotgun, killing Brianna Benlolo, 21, of College Park, and Tyler Johnson, 25, of Mount Airy, Maryland, police said.
Police have found no evidence that Aguilar knew the two victims who worked at a clothing and skateboard shop at the mall, Howard County Police Chief Bill McMahon told a news conference.
''We have no known relationship between the victims and our shooter,'' he said.
Aguilar fired six to eight shots from a 12-gauge shotgun, killing Brianna Benlolo, 21, also of College Park, and Tyler Johnson, 25, police said.
Both were employees of Zumiez, the shop where the shooting took place. A third person on a lower floor of the mall suffered a gunshot wound to the foot, and four more people were hurt in the ensuing chaos.
The attack was the latest in a spate of shootings in recent weeks across the United States that have renewed questions about the vulnerability of public places like shopping centers, schools and cinemas and sparked fresh calls for stiffer gun control.
US Representative Michael McCaul, a Texas Republican and chairman of the House Committee on Homeland Security, said it highlighted the vulnerability of malls.
''You can't have a security lockdown at our malls,'' he said in an interview with CBS.
''It's very difficult to stop a lone gunman who may have mental issues ...(and) wants to kill people. I mean, you can only do so much to stop that.''
But New Jersey Democratic Representative Frank Pallone said shootings underscored the urgent need for better gun controls, including universal background checks, limitations on the sale of ammunition and an assault weapons ban.
''My concern is that this becomes such a norm that people think, 'Well, that's the way it is and nothing can be done,''' Pallone told MSNBC.
The shooting in Maryland followed earlier gun violence at a New Jersey mall in November in which a gunman fired at least six shots without hitting anyone, sparking a mass evacuation of the complex, then killed himself.
The past week saw a student shot dead at South Carolina State University in Orangeburg on Friday, after a teacher's assistant was shot and killed at Purdue University in Indiana on Tuesday.
Suspects have been charged with crimes in both cases.The South Carolina shooting marked the 36th school shooting since 20 children were gunned down at Connecticut's Sandy Hook Elementary School in December 2012, according to the group Mayors Against Illegal Guns.
The Sandy Hook shooting renewed the national debate on gun laws, though gun control measures proposed by President Barack Obama in its wake were rejected by the US Congress.
The shooting happened around 11.15am on Saturday (5.15am Sunday NZT) on the mall's upper level just above the food court. The mall, which has more than 200 stores, was crowded with weekend shoppers; many took shelter after hearing the shots or seeing people fleeing.
Police, who were in the area on another case, responded to the emergency calls within two minutes and found all three bodies either in the store or just outside it.
Aguilar had a backpack filled with crude explosives, and authorities were initially concerned he had booby-trapped himself, McMahon said.
"These were homemade devices consisting of flash powder and household items, so they're not very sophisticated," McMahon said.
Police searched Aguilar's home where they found more ammunition, and seized computers and documents. Police believe that Aguilar legally bought the shotgun himself last month in nearby Montgomery County.
McMahon said he could not yet give further information about Aguilar, including whether he worked or was a student.
College Park is home to the University of Maryland's flagship campus.