Arlington County Commonwealth's Attorney Helen F. Fahey said yesterday that "criminal prosecution is not warranted" in an incident involving former deputy sheriff Ronald B. Hager and his purchase of a truck sold at a sheriff's auction while he was an employe.

Fahey said in a statement that her decision was based on a state attorney general's opinion and "the fact that Mr. Hager no longer works for the sheriff's department and the fact that civil proceedings have been instituted to recover the truck."

The opinion by Attorney General Mary Sue Terry said county employes are prohibited from taking part in these auctions, but there is no criminal penalty in Virginia statutes for doing so. Such actions are also not covered by Virginia's conflict-of-interest law, said Terry.

The question of whether the truck's purchase was illegal was raised by Arlington Treasurer Francis X. O'Leary Jr., a prominent Democrat, who said his department would continue to pursue a civil case against Hager seeking return of the truck and $25,000 in damages.

The truck purchase is an issue in the acrimonious race for Arlington sheriff. Hager, a Republican-backed independent, is challenging Democratic Sheriff James A. Gondles Jr. Hager has accused his former boss of mismanagement and unethical conduct, allegations Gondles has denied.

Hager acknowledges that in 1985 he bought the truck for $25 from a man who bought it for the same amount at a sheriff's auction. The truck is valued at $1,312. Hager denies that the original buyer was acting as a middleman.

Hager claimed vindication yesterday. "I have said all along that I haven't done anything illegal and unethical," he said. "I feel the ruling . . . removed Gondles' one issue against me in this campaign."