The owner of a Montgomery County dry cleaning store was shot and fatally wounded last night after chasing a man who robbed the store at gunpoint, county police said. A 25-year-old Derwood man was later arrested and charged with murder.

Police said a man armed with a gun entered the Nu-Look One Hour Cleaners at 7213 Muncaster Mill Rd. in Derwood about 7 p.m. and announced a robbery. Police said the owner, Patrick Joseph O'Sullivan, 50, put an undisclosed amount of cash into a paper bag and the gunman fled from the store to a car parked across the street.

O'Sullivan gave chase in his pickup truck until both vehicles stopped on a dead-end street, police said. They said the gunman got out of his car, walked over to the truck and fired several shots through the window, striking O'Sullivan in the chest.

O'Sullivan was taken to Shady Grove Adventist Hospital where he died at about 8:30 p.m., police said.

Police said O'Sullivan's son, who had been working in the store, was in radio contact with his father during the chase and was relaying information to police over the telephone.

An off-duty Montgomery County police officer who was in a cruiser and heard a description of the suspect, arrested a man sitting in a car near Rte. 124 near Laytonsville. Police said the stolen money and a handgun were later recovered.

Police said Gregory William Wamsley of 6813 Garrett Rd., Derwood, was charged with murder and armed robbery and was being held at the Germantown police station last night.