Lisa Renee Triggs, the 15-year-old Dale City youth whose body was found in a creek Saturday afternoon, was struck and killed by a blunt instrument, according to Prince William County police, who are investigating the death as a homicide.

Police launched a homicide investigation after an autopsy performed yesterday at Fairfax County Hospital, where Triggs' body was taken after it was found in the creek a quarter-mile from the Center Plaza shopping center.

The autopsy revealed that the youth was killed by a blow to her body apparently inflicted with a blunt object, a police spokesman said.

The spokesman declined to release further details.

The spokesman, Officer Samuel Walker, said there is no suspect in the case and no known motive. He said investigators are searching the neighborhood where the apparent slaying occurred, looking for physical evidence and questioning residents.

He said the apparent slaying occurred in the vicinity of the Bel Air Plantation, which is not a high-crime area.

Triggs, a sophomore at Osbourn Park High School near Manassas, had been shopping at the Center Plaza with her mother and younger brother Wednesday evening when she persuaded her mother to allow her to walk home so she could be on time for her boyfriend's arrival, according to the police account of the incident.

Triggs' mother reluctantly gave her the keys to their home, and the youngster took a shortcut home by walking through a wooded area near the store, police said.

Triggs' parents informed Prince William County police of their daughter's disappearance at 10:50 that night.

Her body was found at about 2:15 Saturday afternoon lying face down in the shallow creek bed about 75 to 100 yards from the path Triggs apparently had taken.

The body was discovered by one of about 70 people who had been combing the area since Thursday.