Twelve people were killed when a public works employee opened fire at a municipal building in Virginia Beach on Friday.

The suspected shooter, a 15-year veteran of public works and a civil engineer, was also fatally shot after a standoff with police.

The shooting, the nation's deadliest of the year, unfolded Friday afternoon around 4 p.m. when the gunman shot the first victim outside Building 2, a three-story brick structure with about 400 municipal workers.

The suspect then headed into the building, moving between floors and firing at workers. Victims were later found on all three floors.

Police are still investigating and have said they do not know the shooter's motive.

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Here's what we know Monday:

What happened?

Police say the suspect was armed with .45 caliber handgun with a silencer, believed to be used for stealth and to keep victims from knowing where the attacker was in the building.

Survivors of the attack said they were caught off guard and initially puzzled by what was happening. One described hearing something that sounded like a nail gun. The gunman brought numerous extended magazines loaded with ammunition, which carry more rounds than traditional magazines, police said.

Gun suppressor used: Suspected Virginia Beach shooter used a gun suppressor. Did it make Friday's shooting deadlier?

The shooting began around 4 p.m. Friday, and by 4:43, police said over their radio that the suspect was in custody.

The gunman was killed in an "intense firefight" with four responding officers who hemmed him in and likely prevented him from using his load of ammunition on more victims, Virginia Beach Police Chief James Cervera said.

One police officer was shot in the stomach during the standoff, but was saved from serious injury by his bulletproof vest.

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The four officers had scoured the building, searching for the shooter. They discovered him on the building's second floor and engaged him in a gunbattle. The suspect fired numerous shots through an office door and wall toward the officers before the gunfire stopped, Cervera said.

That's when officers decided to enter the office and engage the shooter. "That’s not an easy decision to make," Cervera said. "There’s an individual on the other side of the door. He has a gun. He just shot one of your officers."

Police found the gunman alive but wounded and began administering first aid. The suspect died en route to the hospital.

'Officer hit! Officer hit!': How the dramatic Virginia Beach shooting unfolded

Who are the victims?

Eleven of the 12 victims killed in the nation's deadliest shooting of the year worked for the city.

Family and friends remembered some of the victims as long-time public works employees, a vocalist with a "beautiful singing voice," "a super sweet lady" looking forward to a free spa day, a lover of Scottish music who played the bagpipes and a caring neighbor who kept his lawn in great shape.

'Woman of God,' Scottish music lover: These are the victims of Virginia Beach shooting

All were honored at vigils around the city over the weekend.

Four other people were also shot in the attack but survived. All had multiple operations and were recovering at two area hospitals.

Who is the suspect?

The alleged gunman put in his two-weeks notice Friday morning, hours before the attack.

Virginia Beach City Manager Dave Hansen said the man's job performance was "satisfactory," he did not face any disciplinary measures before he notified the city that he intended to quit, and he was not fired.

More: Virginia Beach gunman baffles authorities; suspect was in 'good standing' in his job

"He was in good standing within his department, no issue of discipline ongoing," Hansen said.

The city's Communications Director Julie Hill told the Associated Press on Monday that the gunman's resignation email was brief, unremarkable and didn’t mention anything related to the shooting.

In an interview with USA TODAY, Cervera said the gunman’s motive is a mystery to authorities – a glaring question that may never be answered.

"Right now, we’re open to everything on his motive. We haven’t focused in on any one thing," he said. "You have a professional life, you have a personal life. Is it something that happened in his personal life? Is it something long-term that happened in his personal life?"

The suspect served in the Army National Guard, according to a newspaper clip from the time of his city hire, and received basic military training and advanced individual training at Fort Sill, Oklahoma.

The Federal Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) said he legally bought two .45-caliber handguns – in 2016 and 2018. Both were found at the crime scene, and neither was used in any other crime, the ATF reported.

Contributing: Christal Hayes, Joel Shannon, George Petras, N'Dea Yancey-Bragg, Chris Woodyard, Jeffrey Schwaner, Joey Garrison, Doug Stanglin, Chris Quintana, Mike James, Rose Velazquez, Brad Zinn and Kristin Lam.

This article originally appeared on USA TODAY: Virginia Beach shooting: What we know about the motive, the victims