Authorities are reviewing security footage from the pre-dawn rampage at a New Jersey supermarket Friday, where an ex-Marine killed two coworkers before shooting himself, officials said.
They are also trying to find out where the gunman obtained his arsenal of weapons.
Officials say Terence Tyler, 23, shot through the glass windows of the Old Bridge Pathmark with an AK-47, shooting two coworkers in the aisles before turning a .45 caliber handgun on himself.
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Authorities are checking the footage and investigating whether the AK-47 jammed after he fired 16 shots, since Tyler had two unused rounds of ammunition on him in the store.
Christine Lo Brutto, 18, and Bryan Breen, 24, were killed. Middlesex County Prosecutor Bruce Kaplan said it does not appear they were specifically targeted.
"I believe everybody in the store was a target," he said at a news conference Friday morning.
Lo Brutto, Breen and Tyler had all been working the night shift at the Old Bridge Pathmark, Kaplan said.
A law enforcement source familiar with the investigation said Tyler got into an argument with coworkers; at about 3:30 a.m., he left the store, went to his car and drove away. According to Kaplan, Tyler returned about 20 minutes later, dressed in camoflauge clothing and holding an AK-47.
Tyler shot an employee standing outside the store, and the employee ran inside to warn his co-workers.
Tyler proceeded into the store, firing his weapon, Kaplan said.
A manager "told everyone to run, for their safety for everyone to run, so they all ran to the back, hid for a while," said Miranda Miranda, who has worked at Pathmark, but was not there at the time of the shooting. "When the coast was clear, they left through the emergency exit."
Dragan Jovanovic, the manager of the Staples next door to Pathmark, said the employee who fled told him Tyler threw a shopping cart through the store window, and then started shooting.
"He heard a loud bang, then he ran out the back door," Jovanovic said of the shaken employee.
Employees saw what he was doing and locked the door, a law enforcement source said, but he shot out or broke the windows and went inside.
Kaplan said Tyler stopped at an aisle and fired at five workers, hitting and killing two of them.
Tyler then drew a handgun and shot himself.
Tyler, formerly of Brooklyn but living in Old Ridge, served in the U.S. Marine Corps from 2008 to 2010, a Marines spokesman said. Tyler was a lance corporal and rifleman, who received two medals and never served overseas, the spokesman said.
Tyler had worked at the store for around two weeks, his uncle, Christopher Dyson, told NBC 4 New York. Dyson, who also works at Pathmark and helped get his nephew the job, offered an apology to the victims.
"We don't know what triggered it," said Dyson, his eyes filled with tears.
Police after the shooting found a third weapon -- a shotgun -- inside Tyler's vehicle, and additional ammunition at his home, according to two law enforcement sources. Investigators are looking into how Tyler obtained the weapons.
A vigil for the two victims, both graduates of Old Bridge High School, was planned for Friday night.
"Why does an 18-year-old have to lose her life? She just [began] it," said Carolyn Anders, a neighbor of Lo Brutto. "Heart of gold, always smile on her face."
In a statement, Lo Brutto's family asked for privacy and said they would not be commenting further.
"Cristina was always a positive influence within the school," said Old Bridge High School Principal David Cittadino. "When tragedy strikes like this, it reminds you what's important."
In tweets sent out about four hours before the shooting, Lo Brutto wrote, "Im glad im at work a half an hour early."
Twelve to 14 people were working in the store during the gunfire, Kaplan said. Some workers ran to the back of the store to escape, the source said.
Tyler may have had a history of depression or mental illness, a law enforcement source said.
He was discharged from the Marines in 2010, nearly two years in the service at Twentynine Palms, Calif., according to the Marines. The infantryman never served overseas, said Marine spokeswoman Capt. Kendra Motz. She wouldn't comment on the circumstances of his discharge.
Multiple magazines were found in the store, Kaplan said, and it appeared 16 shots were fired in more than one spot. No shots were fired by police, who did not arrive until the shooting was over, he said.
Authorities were called to the Pathmark on Route 9 in Old Bridge at about 4 a.m. The store was closed at the time of the shooting. It had been scheduled to open at 6 a.m.
Employees who were in the store at the time of the shooting congregated later in the morning outside a TGI Friday's restaurant in the shopping center, where SWAT teams had set up a command post.
In a statement Friday, Pathmark spokeswoman Marcy Connor said, "We express our deepest sympathies to the families of the victims and our appreciation to local law enforcement. Our main concern is the safety of our associates and customers. We continue to work with local authorities during the investigation."