After an unremarkable career spent tending the water and sewer systems beneath this city of 450,000 residents, DeWayne Craddock quit on Friday morning.

The engineer for Virginia Beach's municipal government informed his bosses in an email that he was resigning, city officials said. But Craddock made one last visit to the drab brick building where he worked and methodically killed his colleagues.

Investigators, Craddock's former co-workers and residents of this stricken oceanside community continued Sunday to grasp for clues to what precipitated the deadliest mass shooting in the United States since November. Armed with two .45-caliber pistols, at least one of them equipped with a sound suppressor and extended magazine, Craddock killed 12 people before dying in a gunfight with police.

Some killers leave behind manifestos, YouTube videos or social media profiles that display a mind moving inexorably toward violence. What Craddock left was a resignation letter, according to city officials, and a work history that gave no hint of his intentions.

"Right now we do not have anything glaring," Police Chief James Cervera said at a news conference Sunday. He cautioned that investigators are still trying to determine a motive.

Officials would not discuss what Craddock wrote in his resignation, but a person familiar with the email said it was short and there was "nothing out of the ordinary."

"He merely submitted his two weeks' notice," said the person, who spoke on the condition of anonymity because the investigation is ongoing. "There was no foreshadowing of anything that resulted the rest of the day."

City Manager Dave Hansen said "a very thorough review" of Craddock's personnel file had revealed no problems. "To my knowledge, the perpetrator's performance was satisfactory," Hansen said, adding that Craddock "was in good standing within his department . . . there were no issues of discipline ongoing."

The mystification behind that coolly bureaucratic assessment is shared by others who knew Craddock. A 40-year-old veteran of the Virginia Army National Guard, in which he served for six years as an artillery cannon crew member, Craddock had a shaved head, bodybuilder's physique and cameras in the windows of his house on a peaceful cul-de-sac.

Christi Dewar, a public utilities account clerk for the city who worked across the hall from Craddock, said she knew of no incidents or arguments involving Craddock in the days before the shooting.

"There was absolutely no warning whatsoever," Dewar said.

A 2003 report in Dolan's Virginia Business Observer said Craddock had joined a consulting firm called MSA and was a project engineer. Firm partner Scott Acey declined to comment Saturday beyond confirming that Craddock worked for the company for three to four years before going to work for Virginia Beach.

Shortly before Craddock began Friday afternoon's massacre, he was in the office bathroom, brushing his teeth, said city engineering technician Joseph Scott, who recalled exchanging pleasantries with him.

"Hi, how you doing? Any plans for the weekend?" Scott recalled saying. "That was pretty much it. Then we went our separate ways."

Not long afterward, about 4 p.m., officers received calls about a shooter in Building 2 of the Virginia Beach Municipal Center, a cluster of government offices just east of a golf course in this sprawling city that has a suburban feel. The police headquarters is at the municipal center, and two detectives quickly entered the building, followed by a pair of K-9 officers.

On Sunday, Cervera spoke of the challenges the officers faced as they ventured inside the building, a hivelike complex that was built in the 1970s and whose interior has since undergone additions and renovations.

"It's a maze where the workers are," Cervera said.

Without a clear idea of the shooter's whereabouts, officers entered that maze, eventually encountering Craddock on the second floor. A protracted firefight ensued, and at 4:19 p.m. one officer was hit but was saved by his protective vest. Craddock, cornered in an office, began firing through the door.

Police ultimately broke down the door and began trying to save the life of Craddock, who was wounded. He was taken to a hospital and died.

Between the time police were called to the scene and the time Craddock was subdued, 36 minutes had passed, officials said.

Craddock had killed 12 people - among them the mother of a 22-month-old, civil servants at the end of three-decade careers, a bagpipe enthusiast, an immigrant from Belarus who helped his friend with yardwork - in what authorities described as an indiscriminate killing rampage.

His first victim was a contractor in the parking lot, there to file a permit. The second was a woman on her way out of the office. Craddock, who was still an active employee, was able to use his badge to gain access to the building's second floor, where most of the killings occurred.

Within about an hour of the initial attack, Cervera said, all of the wounded had been transported for emergency care. In addition to those killed, four people were wounded and hospitalized. All have undergone surgeries, officials said Sunday.

"They are progressing, and our prayers are with them," said Hansen, the city manager.

Authorities identified those killed as Virginia Beach residents Michelle "Missy" Langer, Ryan Keith Cox, Tara Welch Gallagher, Mary Louise Gayle, Alexander Mikhail Gusev, Katherine Nixon, Joshua Hardy and Herbert "Bert" Snelling; Chesapeake residents Laquita Brown and Robert "Bobby" Williams; Norfolk resident Richard Nettleton; and Powhatan resident Christopher Kelly Rapp.

Officials said they were still piecing together Craddock's whereabouts and timeline on Friday. It was not clear when and where he picked up the guns before the massacre began. Officials with the Bureau of Alcohol, Tobacco, Firearms and Explosives said his two pistols were legally purchased in 2016 and 2018.

They found another legally purchased firearm at his home and a second gun that investigators were working to trace. At the scene of the shooting, police found a sound suppressor, commonly known as a silencer, and extended magazines.

At Sunday's news conference, city officials disclosed that Craddock resigned Friday morning but did not share details of how or why he did so. "He notified his chain of command that morning," Hansen said. "My understanding is he did that via email."

The municipal center will remain closed Monday, but Virginia Beach's other city offices will be open. All Municipal Center buildings other than Building 2, where the attack took place, will reopen Tuesday.

"Our recovery is underway," Hansen said. "Our grieving is underway."

But as church services and vigils were held Sunday across Virginia Beach, the state's largest city and a tourist destination where jets from the naval air station regularly roar overhead, many residents said they were still struggling to come to terms with what had happened.

Hundreds gathered Sunday morning near the iconic Neptune sculpture on Virginia Beach's sandy coast for an evangelical service hosted by Trinity Church.

Patty Richards said she had been praying since she heard that people had been shot at the municipal center not far from her house. Her husband, a contractor, had planned to go to that building on Friday for a permit but put it off until Monday.

Richards, a former critical cardiac nurse at a nearby hospital, said she understood too well what the medical professionals treating the victims would be facing.

"I know about that kind of pain," she said.

Cassandra Ellis, 47, stopped at the grocery store with her boyfriend for a dozen roses - one for each victim - adding them to the stack of flowers left in front of the Police Department.

"I go on my Facebook feed and that's all there is," Ellis said. "The vibes, the emotion is totally different. We're in a deep place, a bad place."

It was the deadliest shooting in the city's history, and the latest in the series of American gun massacres that now arrive with numbing frequency. The Virginia Beach shooting was the deadliest in the United States since a November shooting at a bar in Thousand Oaks, California, in which 13 people, including the gunman, were killed.

Robert Richardson, 39, a retired Navy veteran, said he had been drawn to the tightly knit community in Virginia Beach. "It has a big-city feel, but it doesn't act like a big city," he said.

On Sunday, Richardson brought his four children to see the flowers at the police station. "My kids go to school a block down the road," he said. "I fear for my kids when they go to school."

Richardson said he lived through the shooting at Fort Hood, Texas, in 2014 and never thought he'd see another workplace slaying like that one.

Now that more have taken place, he can't say he's surprised.

"If we didn't learn from Sandy Hook, where all these innocent children died, we'll never learn," he said.

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The Washington Post's Moriah Balingit, Rachel Chason, Alice Crites, Jim Morrison, Gregory S. Schneider, Julie Tate and Matt Zapotosky contributed to this report.