Fairfax County's chief prosecutor ruled yesterday that there was no criminal liability on the part of a county police officer who fatally shot an unarmed suspect during an undercover drug operation last month.

Commonwealth's Attorney Robert F. Horan Jr. said police investigator Irene M. Boyle will not be prosecuted, but also said he disagreed with the police department's findings that the Nov. 16 shooting was accidental.

"I don't believe it was an accident," Horan said yesterday. "She saw his arms come up, she thought he had a gun and she fired."

Police said Jose Carlos Rodriguez, the slain suspect, startled investigator Boyle, and an "unintentional reflex" caused the officer's gun to discharge. Police said Boyle had no intention of firing.

Police said the suspect was sitting in a parked truck and negotiating a drug sale with an undercover officer. Boyle was covering the undercover officer from a cruiser. On a signal from the undercover officer, Boyle walked toward the truck with her gun drawn. As she came within a foot of the truck, Rodriguez's face suddenly appeared in the window of the cab, police said.

"He kind of came up like a jack in the box," Col. John E. Granfield, chief of police, said after the shooting. "It startled her, and she shot the gun."

Police spokesman Warren Carmichael said yesterday that although the interpretations of what happened are slightly different, the bottom line is the same. He said Boyle, an 11 1/2-year veteran, would remain on administrative duty until the department determines whether police policies and procedures were followed.

Rodriguez's family members could not be reached for comment on Horan's ruling.