Investigators have identified two .45 caliber pistols used in a mass shooting at a municipal building on Friday that killed 12 people, a US official said Saturday.

All indications were that the guns were bought legally, Ashan Benedict, regional special agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives, a federal law enforcement organization within the US Department of Justice, told a press conference.

One gun was bought in 2016 and the other was bought last year, according to Benedict.

He also said two other weapons were found at the home of the gunman, identified earlier in the day as 40-year-old DeWayne Craddock, a city engineer who had been employed with Virginia Beach's public utility department for the past 15 years.

The shooting massacre that the suspect carried out on Friday afternoon led to 12 fatalities and four injuries. He was killed in a "long gunbattle" with police officers.

US President Donald Trump has ordered US flags to be flown at half-staff in memory of the victims.

In a statement released by the White House on Saturday, Trump said that he was ordering the action as a "mark of solemn respect for the victims of the terrible act of violence."

Those who were killed by the gunman include 11 city employees and the other was a contractor seeking a permit, City Manager Dave Hansen told reporters Saturday morning.

"I have worked with most of them for many years," he said. "They leave a void that we will never be able to fill."

Vigils in memory of the victims were planned for Saturday evening.

Virginia Beach is a popular resort city with an estimated population of some 450,000, which is about 300 kilometers south of Washington, D.C.

The shooting occurred in Building No. 2 of the sprawling Virginia Beach Municipal Center, which includes several city facilities, including the police department.

Building No. 2 houses offices for planning and public works and is adjacent to city hall.