A former D.C. police lieutenant, once honored as the department's officer of the year, was sentenced yesterday to four months in prison after admitting that he took $8,000 in bribes from the operators of illegal massage parlors.

Yong H. Ahn got a break on his sentence because he began cooperating with authorities in a broader corruption investigation immediately after his arrest in February 1998. Defense attorney Jonathan Shapiro said Ahn helped authorities obtain a guilty plea from a "significant official in the D.C. government." Details about that case remain under seal at U.S. District Court. Authorities would not identify the official or say whether that person is still a city employee. The investigation is continuing.

Prosecutors had hoped Ahn would become a valuable long-term witness and would lead them to wrongdoing within the D.C. government. They initially attempted to keep news of his arrest a secret so he could do undercover work. Ahn wore a recording device for a short while, and he met with potential targets of the probe. But news reports began circulating two months after Ahn was apprehended and short-circuited their investigation, they said.

U.S. District Judge Thomas F. Hogan said Ahn deserved credit for quickly pleading guilty to a charge of accepting an illegal gratuity and for aiding investigators. "I find that through no fault of your own, you could not provide more substantial assistance," Hogan told Ahn yesterday.

Both prosecutors and defense lawyers had asked Hogan to depart from federal sentencing guidelines, which would have netted Ahn a prison term of eight to 14 months. Hogan gave Ahn four months in prison, followed by four months of home confinement. Hogan said the prison term was essential so that Ahn can "stand as an example to other police officers."

Ahn, 42, was the police department's highest-ranking officer of Korean heritage. He had won dozens of awards and commendations during his 15 years on the force and was hailed as an early advocate of community policing. At the time of his arrest, he was a supervisor in the 1st Police District.

According to documents unsealed yesterday, Ahn's arrest came after authorities got a tip that a police official was taking bribes from Asian-operated massage parlors that allegedly provided prostitution services as well as innocent rubdowns. After identifying Ahn as a suspect, authorities put him under surveillance and enlisted the help of two Northwest Washington massage parlor operators.

Ahn went to both massage parlors and told the operators the establishments weren't licensed, prosecutors said. The operators gave him a total of $8,000 on four occasions between September and January 1998, prosecutors said, and at least some of the payments were videotaped.

Besides Ahn's activities, prosecutors said, investigators learned "numerous massage parlors were paying money" to an employee of the D.C. Department of Consumer and Regulatory Affairs, the agency that issues business licenses and building permits. In return for cash, the massage parlors were given certificates of occupancy that identified them as health spas or exercise spas, making them less likely to face police scrutiny, a source said.

Assistant U.S. Attorney Robert R. Chapman urged Hogan to impose some prison time. In sentencing papers, Chapman wrote, "The fact that the victims were operators of illegal establishments and not particularly sympathetic does not diminish the defendant's culpability."

Ahn offered no explanations in court yesterday.

Ahn had one previous brush with trouble. He was charged in 1992 with shoplifting at a Northern Virginia store. The criminal charges eventually were dropped but a police trial board found Ahn guilty of misconduct and urged his firing. Then-Police Chief Fred Thomas decided the trial board's recommendation was too harsh and instead suspended Ahn for 60 days.

Ahn, who left the police department and now runs a dry-cleaning business with his wife, attempted to withdraw his guilty plea last year, but Hogan wouldn't permit it. He said Ahn can remain free on bond while his attorney asks the D.C. Circuit Court of Appeals to review the case.