READ: Virginia Beach Mass Shooting Suspect's Resignation Letter Hours Before Killing 12

A resignation letter written by DeWayne Craddock gives no clear indication why this Virginia Beach government employee described by some of those who knew him as quiet and polite would murder 12 people in the worst non-terrorist mass workplace killing in U.S. history.

Craddock, an African-American, was a certified professional engineer who worked for the Virginia Beach public utilities department for 15 years. He was 40 at the time of his death.

His resignation letter made public on Monday was typical as far as resignation letters go.

"I want to officially put in my two (2) weeks' notice to vacant (sic) my position of Engineer III with the City of Virginia Beach. It has been a pleasure to serve the City, but due to personal reasons I must relieve my position," wrote Craddock.

In response, a supervisor wrote Craddock: "I hope you are able to resolve your personal reasons. To be clear, your last work day will be Friday, June 14, 2019."

"Thank you. Yes, that is correct," replied Craddock.

City Manager Dave Hansen clarified that Craddock wasn’t fired or forced to resign. Craddock was in "good standing" and had no ongoing discipline issues.

Hansen’s appraisal of Craddock jibes with the judgment’s of other people who knew Craddock. A fellow employee described Craddock as a “nice guy” who was quiet and polite.

Joseph Scott, a city engineering technician, who knew Craddock, said he ran into Craddock at the men’s restroom of the building minutes before the shooting. After the murders, Scott says he was in complete disbelief after learning later Craddock was the shooter.

At a vigil on Saturday morning, Scott said he prayed not only for the victims but also for Craddock.

Craddock’s murderous rampage on the afternoon of May 31 began at the parking lot of Building 2 of the Virginia Beach Municipal Center where he shot dead a contractor sitting in his car. He then entered the three-storey building and shot people on all three floors. He killed 12 persons and wounded 4 others.

The gunman was later killed by city police officer after a prolonged gun battle inside the building. He was armed with two .45 caliber automatic pistols loaded with extended magazines.

Police say Craddock fired indiscriminately and there is still no clear indication he was targeting anyone in particular. His resignation letter does not mention a beef with any of his managers or fellow workers.

Craddock’s crime is the worst non-terrorist workplace shooting in the U.S. In number of fatalities, it is exceeded only by the 14 persons murdered by Syed Rizwan Farook and his wife, Tashfeen Malik, during a murder spree in California in 2015.

The Farooks massacred 14 people and wounded 22 others at an office party of his colleagues at the Inland Regional Center for San Bernardino County health workers. Police say Farook was inspired by Islamist militants. The couple was killed in a shootout with police.