Montgomery County's second-ranking prosecutor was arrested and charged with drunken driving Saturday night after a police officer spotted her county-issued car moving unusually slowly on a Bethesda road, police said yesterday.

Katherine S. Winfree, the county's principal deputy state's attorney since 1999, was charged with driving under the influence, driving while impaired and driving an unsafe vehicle. Her car had a flat tire, police said.

The officer "detected the odor of alcoholic breath" after Winfree was pulled over in the 9200 block of Fernwood Road about 11:55 p.m., said police spokeswoman Joyce Utter. Utter refused to say whether Winfree had taken sobriety tests, noting that the case was still under investigation.

Police did not provide a copy of the arresting officer's report, but a source familiar with the investigation said Winfree had declined to take a breath test.

State's Attorney Douglas F. Gansler successfully lobbied the General Assembly last year to change the law so that prosecutors could tell a jury that a defendant refused to take a sobriety test. Prosecutors worry that drivers sometimes use their right to refuse a breath test as a way to sidestep police efforts to gather evidence.

Winfree did not return a call for comment. Her boss, Gansler, did not return several phone calls. His office has forwarded the case to Frederick County for prosecution.

Gansler wanted to avoid any perceived conflict of interest from "this office prosecuting one of its employees," said John McLane, spokesman for the state's attorney office.

McLane declined to answer questions about specifics of the case, saying only, "We're confident that the case will be treated like any case of its type and the appropriate resolution will be reached."

McLane said he did not have available the officer's report, which is usually given to prosecutors in DUI cases and usually contains details about any sobriety tests.

Winfree's caseload consists of serious criminal cases -- including last year's prosecution of a man who killed a Catholic priest and her recent involvement on the sniper task force -- rather than traffic cases, such as DUI charges. But when she was an assistant U.S. attorney in the District, one of her highest-profile cases was the successful prosecution in 1997 of a Georgian diplomat who killed a teenage girl in a drunken driving crash and tried to claim diplomatic immunity.

The Maryland Attorney Grievance Commission, which investigates complaints about lawyer conduct, will likely review Winfree's case. The commission generally looks into all arrests of lawyers and can recommend sanctions to the state's Court of Appeals.

F. Patrick Kelly, former chairman of the Montgomery County Bar Association's criminal law section, said the arrest should not preclude Winfree from continuing to do her job. "Having one DWI case doesn't mean someone has an alcohol problem," Kelly said. " . . . That is not enough to say she shouldn't prosecute cases."

Kelly said Winfree is known as a tough but fair prosecutor.

"That's a shame," Kelly said of Winfree's arrest. "I've seen her exhibit nothing but competency and professionalism."

Staff writer Matthew Mosk contributed to this report.