The rampage in Virginia's biggest city was the worst mass shooting in US since November 2018 (File Photo)

Officials trying to learn what prompted a city engineer to kill 12 people in Virginia Beach before he was shot dead by police said on Sunday investigators were examining employment-related issues but stressed that the suspect was not being disciplined.

Recounting the chaotic minutes that followed the Friday afternoon gun rampage, authorities said they were still seeking a motive for why DeWayne Craddock, 40, would launch the attack on his co-workers and others at a municipal complex.

"To the extent that the subject's employment status has anything do with these events, that will be part of the ongoing investigation," City Manager Dave Hansen told reporters. "However, he was not terminated and he was not in the process of being terminated."

Craddock, an employee in the coastal resort city's public utilities department for about 15 years, had been described by officials as disgruntled. But Hansen contradicted reports that he was being disciplined or fired.

"To my knowledge, the perpetrator's performance was satisfactory, that he was in good standing within his department and that there were no issues of discipline ongoing," he said.

On the morning of the shooting Craddock tendered his resignation by email, but he had not been under pressure to do so, Hansen added.

Using his employee pass to enter secure areas of the building where he worked, Craddock began indiscriminately shooting people with two .45 caliber pistols, reloading with extended ammunition magazines, according to police.

All but one of those killed were employed by the city, while the other was a contractor seeking a permit, officials said.

The rampage in Virginia's biggest city was the worst mass shooting in the United States since November 2018, when a dozen people were slain at a Los Angeles-area bar and grill by a gunman who then killed himself.

Three of the four people who were wounded were listed in critical condition on Sunday, while the fourth was critical but stable. Officials said all had undergone surgery.

A police officer who was shot but saved by his ballistic vest has been treated and released.

The 11 dead municipal workers had been employed for between 11 months and 41 years. Six worked in the public utilities department and five were employed in the public works department.

Virginia Beach Police Chief James Cervera said officers were on the scene within two minutes of the first report at 4:08 p.m. EDT (2008 GMT) and that the suspect was taken away within 36 minutes.

With police entering the maze-like 1970s era Building Two of the complex from several points, four officers from different commands worked together to locate the suspect on the second floor "and immediately engaged in a gun battle," Cervera said.

During the exchange of fire, with the number of rounds fired by Craddock running "well into the double digits," Cervera said the suspect led the officers on a chase through office doors that he was able to access and that locked behind him.

"They were returning fire, and at one point the suspect was firing through the door and the walls at the officers," he said.

Cervera said the officers found the suspect alive and administered first aid, but that he died after being rushed to a hospital.

Stressing that the investigation is still in progress, Cervera said police do not believe the suspect was targeting any specific individual.

A federal agent said on Saturday that the handguns used in the attack were bought by the shooter legally within the past three years, and police said at least one of the pistols was fitted with a "sound suppressor."

The city is planning a memorial service for Thursday evening. All municipal buildings will be closed on Monday, officials said.