Like many veteran civil engineers, DeWayne Antonio Craddock was a methodical man — no less so as he stalked the three floors of his Virginia Beach workplace, at the city Municipal Center, gunning down each co-worker he saw, one by one.

“As he came upon someone, that’s when he took action,” Virginia Beach Police Chief James Cervera said at a Saturday press conference.

He came prepared, officials said, carrying a security pass to reach deep into the building, and multiple ammo clips to extend the firepower of his two .45-caliber handguns, at least one of which was equipped with a sound suppressor.

Craddock would kill 12 people, all but one a co-worker, injure four others, and then die in a police shootout in this year’s worst mass shooting.

But first, the former National Guardsman, described as a tense, terse, solitary man of routine, made a final trip to the men’s room.

“He was in there brushing his teeth, which he always did after he ate,” fellow engineer Joseph Scott said of running into Craddock five minutes before Friday’s 4 p.m. shooting.

“I said, ‘Hey, how you doing?’ ” Scott, who was about to leave for the day, recalled asking his coworker as they stood side-by-side in the men’s room.

“He said he was doing OK. I asked, ‘Any plans for the weekend?’ And he said, ‘No.’ ”

“Well, have a good day,” Scott told him.

“And he said the same to me.”

Just why Craddock opened fire at the city’s Department of Public Utilities Building 2, part of a sprawling municipal complex, where he held a mid-level engineering job for 15 years, remained a mystery a day after the massacre.

He had not been fired, as some early reports had claimed; he left no explanatory note or online manifesto. In fact, he apparently shunned social media.

He had no criminal record, other than a few traffic violations — and both the firearms he used Friday were legally obtained, officials said.

It’s no wonder that City Manager Dave Hansen called the deadly rampage “a senseless, incomprehensible act of violence.”

Craddock’s department director, Bob Montague, told The Washington Post, “There is no answer to explain an event like this.”

Still, an early picture is emerging of Craddock, who graduated in 1996 from Denbigh HS in Newport News, Va., then joined the Army National Guard, training at Fort Sill, Okla.

He served until 2002 and was assigned to a Norfolk, Va.-based battalion as a cannon-crew member. He next studied civil engineering at Old Dominion University, earning a bachelor’s degree.

After a few shorter-term, private-sector engineering jobs, he started at the Virginia Beach ­Department of Public Utilities.

By day, Craddock worked at the department, in a campus of about 30 brick Colonial-style buildings in the rural Princess Anne area.

The campus is also just about 100 to 150 yards from police headquarters.

Craddock and his co-workers labored to keep 35 million gallons a day of clean water flowing to residents and to make sure an equal quantity of sewage and wastewater flowed through the city’s 400 sewer pumping stations to two sewage-treatment plants.

Craddock was regularly listed on city press releases about infrastructure projects and was once quoted in a newspaper article about one of the municipality’s pumping stations.

“The way it works is Norfolk, where the water-treatment facility is located, tells us how much to pump and we pump it to them,” he explained during a tour for the Virginia Beach Chamber of Commerce board in 2015, the Lake Gaston Gazette ­reported.

An online résumé that appears to be Craddock’s listed him as a project manager for Virginia Beach public schools in 2012.

As for hobbies, he was also listed as a runner in a 5K race in Norfolk and as a guest speaker at a North Virginia Beach Civic League in 2016.

He was a member of his neighborhood-association board and spent a good deal of time at the gym, neighbors told reporters.

But mostly he was a loner, preferring to wave and keep walking when he crossed paths with neighbors.

Several neighbors believed that he had once been married, but that his wife had left him some years ago.

“I would speak to him, and he would speak back, but conversation-wise, I never had a conversation with him,” said one neighbor, Angela Scarborough.

He would keep cameras trained on the two nice cars he kept parked out front of his condo, one of them a Mustang, neighbor Cassetty Howerin, 23, told ABC News.

She never saw a guest come to visit.

“He never really cracked a smile,” she said of Craddock, whom she described as “jacked” from his many hours at the gym.

Significantly, within at least the last three years, he began quietly, but legally, amassing firearms, two of which were recovered at Craddock’s condo.

One of those weapons was a legally obtained .45-caliber pistol, officials said. Authorities were still checking out the fourth weapon.

The shooting was the worst in the US since November 2018, when Ian David Long murdered 12 people at a country western bar and grill in Thousand Oaks, Calif., and then killed himself.

With Post wires