COLUMBIA, Md. (AP) — A gunman opened fire at a busy shopping mall in suburban Baltimore on Saturday, sending store employees and customers scrambling for cover. Police said three people died, including the person believed to be the shooter. Five people were injured, none seriously.

Police were still trying to determine the identity and motive of the heavily armed gunman who killed a man and a woman, both in their 20s, at a skate shop called Zumiez on the upper level of the Mall in Columbia, a suburb of both Baltimore and Washington. Their identities were not released.

Howard County Police Chief William J. McMahon said at a news conference that authorities ran into difficulty in identifying the gunman because of concerns that he might be carrying explosives and were proceeding with an "abundance of caution."

McMahon also called reports that the killings were "domestic" in origin "pure speculation."

"We do not know yet what caused the shooting incident," he said. "We do not have a motive."

Someone called 911 at around 11:15 a.m. to report a shooting at the mall. Police responded to the scene within 2 minutes and found three people dead — including one person who was found near a gun and ammunition — either inside or outside the shop, which sells skateboards and clothing and accessories.

Howard County General Hospital said on its Facebook page that it had received four patients for treatment who were in good condition. One patient was reported to be a shooting victim while the hospital said three other patients sustained other injuries.

The mall is at the center of the town and typically opens at 10 a.m. on Saturdays. It was busy with shoppers and employees when shots rang out before noon, sending scores of people scampering to find the nearest hiding place. Many found cover in backrooms of stores and barricaded themselves in until police arrived.

Joan Harding of Elkridge, Md., was shopping with her husband, David, for a tiara for their granddaughter's 18th birthday. She said she heard something heavy falling, followed by gunshots and people running.

"My husband said, 'Get down!' and the girl that worked in the store said, 'Get in the back,' " Harding said. That is where they hid until police searched the mall and signaled it was safe to leave.

At a news conference, Howard County Police Chief William J. McMahon said police are relatively confident that there was only one shooter.

"We don't know a motive yet," McMahon said. "We are very confident that it was a single shooter, and there was not another shooter in the mall."

The mall was closed to the public as police went store to store looking for people who might still be hiding, McMahon said. It will remain closed at least through the night.

McMahon said the shooting occurred at a store on the upper floor. He said it wasn't clear whether the shooting was random or whether the shooter and victims knew each other. While not explicitly calling it a murder-suicide, he said officers did not fire any shots when they arrived at the scene.

Witnesses described moments of panic as they heard a succession of gunshots and screaming as people ran for cover into nearby stores and hid behind locked doors.

Tonya Broughton of Silver Spring, Md., was with a friend getting facials for a 'girls morning out,' she said. "The only thing I heard was all the people running and screaming and saying 'There's a shooter! There's a shooter!' " she said.

Wearing a gel face mask, she and her friend hunkered down in a Victoria's Secret store.

People were directed out of the mall and into a parking lot, where some boarded a bus and others walked toward their cars. Some people were seen crying. McMahon said detectives were interviewing witnesses as they emerged from the mall to try to get a better picture of the events that had unfolded.

Laura McKindles of Columbia works at a kiosk in the mall. She said she heard between eight and 10 gunshots, followed by people running and screaming. She ran into the backroom of a perfume store and locked the door.

Allison Cohen, who works at the apparel store Lucky Brand Jeans, said she always felt safe at the mall.

"I truly never thought something like this would ever happen here," Cohen said. "It's really, really shocking."

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Associated Press reporter Martin Di Caro contributed to this report from Washington.