Virginia Beach, Virginia (CNN) Investigators are still trying to get a better picture of what led a Virginia Beach city engineer to bring two handguns to work on Friday and empty extended magazines in a deadly shooting spree.

At a gathering of some 4,000 city employees Monday afternoon, City Manager Dave Hansen stood on a stage and admitted to his staff that he still didn't know why gunman DeWayne Craddock did what he did, Hansen told CNN.

Craddock killed 12 people and injured others before police killed him in a shootout.

While rumors have circulated among city workers that the gunman was recently disciplined or set to be fired, senior officials in Virginia Beach expressed confidence that Craddock was not on the verge of being forced out of his job.

On Monday, the city released a letter of resignation Craddock submitted on Friday, hours before the shooting. An engineer in the city's public utilities department, he said he was submitting his two weeks' notice "due to personal reasons," adding "it has been a pleasure to serve the City."

Read More