Investigators are trying to locate an armed man who opened fire on a Petaluma gym employee during a robbery attempt Monday evening, police said. Officers responded to a report of an armed robbery at around 6:15 p.m. at Curves, a fitness center for women located at 40 E. Washington St. Witnesses said a man wearing a Halloween mask had entered the gym and ordered a female employee to put money inside a black bag. When she refused, he pulled out a small-caliber handgun and fired one round at her, narrowly missing, according to police. The suspect then ran out the rear door of the gym and entered a gray, two-door Acura. He was last seen heading east on East Washington Street.No one was injured in the robbery attempt, and the suspect apparently fled without taking any money, police said.The suspect was described as a white man with dark, greasy hair, between 18 and 30 years old, standing 5 feet 6 inches to 5 feet 8 inches tall, and weighing about 140 pounds. He was wearing all dark clothing and a Halloween mask over his face.Investigators are asking anyone with information on the crime to contact the Petaluma Police Department at (707) 778-4372.