He allegedly repeated the plot Aug. 30, charging documents say.
Donna Messina, the city's human resources director, said the Water and Sewer Department notified her of the threat Sept. 1. She contacted the city's police department and fire marshal's office.
On Sept. 2, Allison, 26, of 56 Opal Court, Martinsburg, W.Va., was charged with one count of threatening to explode a destructive device in a structure. The charge is a misdemeanor punishable by up to 10 years imprisonment, a $10,000 fine or both.
Asked Thursday if Allison lost his job, Messina would only say, "Mr. Allison is no longer an employee of the city of Hagerstown."
Messina said Allison was a collection system operator I. He was hired in March 2003.
Allison did not return a message left on his cell phone Thursday afternoon.
Charging documents say Allison told Scarlett on Aug. 26 that he "had learned how to make bombs in his high school chemistry class."
On Aug. 30, he repeated the bomb plan to Scarlett, who felt the threat was "valid" and was "imminent" the next day, charging documents say.
Allison reportedly told Scarlett he'd tip him off before exploding the bomb, charging documents say.
Co-worker John Semler also reported hearing Allison make the threat and say he "was going to take care of it his way," a reference to Scriever, charging documents say.
Charging documents say Allison told Hagerstown Assistant Fire Marshal Richard L. Miller on Sept. 1 that he had made the bomb threat and had "wanted to hurt (Scriever) about two weeks ago."
Allison "was angry at his supervisor ... because of his treatment of him," charging documents say.
Allison is due to appear in Washington County District Court on Nov. 1.
The Herald-Mail published a story about the alleged threat Sept. 9. At the time, Fire Marshal Tom Brown said the investigation still was under way. Miller said Thursday that Brown might not have known charges already had been filed.
Charging documents had Semler, Scarlett and Scriever spelled other ways, but Messina provided the spellings listed in city records.