(CNN) At least 12 are dead and at least four others are injured after a shooting Friday afternoon at the Virginia Beach Municipal Center.

Police have said the gunman succumbed to his injuries after a long gunfight with officers. A Virginia government source briefed on the investigation told CNN the shooter was a "disgruntled employee," but police said Saturday morning they were still trying to determine the gunman's motive

Here's what we know:

How the shooting unfolded

The shooting began late Friday afternoon when a man opened fire on all three floors of the municipal center's Building 2, which houses the operations building for the city that includes multiple departments.

Four officers -- two detective supervisors and two K-9 officers -- responded to the reports of the shooting and immediately exchanged fire with the gunman, Virginia Beach police Chief James Cervera said.

"I can tell you that it was a long gunbattle between those four officers and that suspect," Cervera said Friday night.

One of the officers who responded was shot, Cervera said, and was saved by his bullet-proof vest.

The chief said when the shooter went down, the officers performed CPR.

Cervera said victims were found on three floors of Building 2. Municipal center employees described hiding in offices and under desks

The FBI responded to the shooting to aid local authorities.

The gunman was a professional engineer

The gunman was DeWayne Craddock, a 15-year employee of the city of Virginia Beach, Cervera said Saturday.

Craddock, 40, was a certified professional engineer in the city's public utilities department. He is listed on department press releases as a point of contact for information on local road projects over the last several years.

A search of online court records in Virginia Beach and surrounding counties shows Craddock was cited for a motor vehicle infraction in 2013 but nothing else.

Craddock served in the Army National Guard as a private after attending Denbigh High School in Newport News, Virginia, according to a 1996 report from the Daily Press newspaper.

While Craddock was identified as the shooter, Cervera said he would mention his name only once.

"He will be forever referred to as 'the suspect,' because our focus now is the dignity and respect for the victims of this case and to their families," he said.

Police work the scene where 12 people were killed during Friday's mass shooting in Virginia Beach.

Authorities recovered a .45-caliber handgun with extended magazines and a suppressor, the police chief said. A rifle also was found at the shooting site, according to a law enforcement official. Cervera declined to comment about the presence of a rifle.

Two law enforcement sources said the weapons appeared to have been legally purchased. One of those sources said the purchases were made in recent weeks.

Investigators found "additional weapons" at the gunman's home, the police chief said Saturday.

Craddock's parents, reached by phone Friday evening, told CNN they weren't aware of any trouble he was having with this employer.

Before he worked for the city, a Virginia-based engineering firm hired Craddock in 2003 as a project manager. An article in a business newspaper that mentioned the hiring said Craddock held a bachelor's degree in civil engineering from Virginia's Old Dominion University.

According to Virginia's Department of Professional and Occupational Regulation, Craddock obtained his certified professional engineer license in 2008. The license was set to expire in June 2020.

Craddock was 'brushing his teeth like he always did' at work, colleague says

Sometime before the shooting Friday, a co-worker of Craddock's had a final exchange with him that amounted to "have a good weekend," the colleague said.

Joseph Scott, who said he worked with Craddock for several years, saw him in a bathroom at work Friday.

"He was at the sink, brushing his teeth like he always did," Scott told CNN. "I used the bathroom and walked up and was washing my hands, and I said, 'How are you doing?' He said he was doing OK.

"I asked, 'Any plans for the weekend?' And he said, 'No.' And I said, 'Well, have a good day,' and he said the same to me.

"And it was no more than that."

Scott said Craddock was "what I thought was a good person," and described him as generally quiet.

"When we were together, we would talk about family, friends, things that we were going to do, trips we were going to take and things like that," Scott said.

The 12 who were killed were named Saturday morning

Eleven of the 12 killed were city employees. The other was a contractor who was there to fill a permit, City Manager Dave Hansen said Saturday morning.

Hansen said the 11 slain city employees had worked for Virginia Beach anywhere from 11 months to 41 years.

They were Virginia Beach residents Tara Welch Gallagher, Mary Louise Gayle, Alexander Mikhail Gusev, Katherine A. Nixon, Ryan Keith Cox, Joshua O. Hardy, Michelle "Missy" Langer; Chesapeake residents Laquita C. Brown and Robert "Bobby" Williams; Norfolk resident Richard H. Nettleton; and Powhatan resident Christopher Kelly Rapp.

Also killed was the contractor, Herbert "Bert" Snelling, of Virginia Beach.

Nettleton, an engineer with the city's public utilities department, "served with me as a lieutenant in Germany in the 130th Engineer Brigade," said Hansen, the city manager.

Four injured people were being treated at two Sentara Healthcare hospitals. Three were in critical condition, and one was in fair condition Saturday morning, Sentara President and CEO Howard P. Kern said.

A victim is loaded into a helicopter.

"This is a horrific day for the Commonwealth of Virginia," Gov. Ralph Northam said Friday. "Our hearts ache over the senseless violence that has been inflicted upon the Virginia Beach community today."

Mayor Bobby Dyer described it as "Virginia Beach's darkest hour."

"A senseless crime happened that imposed tremendous grief upon the people of Virginia Beach, the commonwealth and this country," Dyer said.

"I believe that our community, Virginia Beach, along with our neighbors and our other cities in Hampton Roads and our commonwealth and our country, we'll be there ... for our families. We're gonna show that Virginia Beach is a city of resolve and dedication."

Correction: A previous version of this story misstated a victim's name. It is Joshua O. Hardy.