After an unremarkable career spent tending the water and sewer systems in Virginia Beach, DeWayne Craddock quit on the morning of May 31, The Washington Post quoted the city officials as saying on Sunday.
The engineer for Virginia Beach's municipal government informed his bosses in an email that he was resigning.
But armed with two .45-caliber pistols, Craddock made one last visit to the building later in the day and opened fire on his colleagues before dying in a gun battle with police.
"Right now we do not have anything glaring," Police Chief James A. Cervera said at a news conference on Sunday.
He added that investigators were still trying to determine a motive.
While the police has not made the resignation letter public, an informed source told The Washington Post that it was short and there was "nothing out of the ordinary".
"He merely submitted his two weeks' notice," said the source, who spoke on the condition of anonymity. "There was no foreshadowing of anything that resulted the rest of the day."
City Manager Dave Hansen said that "a very thorough review" of Craddock's personnel file had revealed no problems.
"To my knowledge, the perpetrator's performance was satisfactory," Hansen said, adding that Craddock "was in good standing within his department".
Craddock, 40, was a veteran of the Virginia Army National Guard, in which he served for six years as an artillery cannon crew member.
Hundreds gathered on Sunday morning near the iconic Neptune sculpture on Virginia Beach's sandy coast for a memorial service for the 12 victims.
This incident was the deadliest mass shooting in the US since November 2018 when 12 people were killed at the Borderline Bar & Grill in Thousand Oaks, California.
( With inputs from IANS )