A Fairfax County judge wasted no time yesterday dismissing a criminal case against a former Prince George's County corrections official, finding the charge against her baseless after watching a videotape that showed she did not, as prosecutors alleged, claim to be a sworn officer.

After reviewing the video, recorded by a camera mounted in a police car, and hearing from an investigating officer, the judge addressed the prosecutor even before former deputy corrections director Rose C. Merchant's attorney could rise from his seat.

"Ma'am, there's no case here," Fairfax General District Court Judge Ian M. O'Flaherty told Assistant Fairfax Commonwealth's Attorney Erin Sylvester. "This case is dismissed."

In February, after Merchant was arrested by Fairfax police on a misdemeanor charge of impersonating a law enforcement official, Prince George's promptly fired her from the civilian job she had held since August 2005. Merchant, 38, a clinical psychologist with a PhD from Howard University, would like to return to the position, her attorney, Darwyn L. Easley, said.

Prince George's officials yesterday declined to discuss Merchant's firing. Vicki D. Duncan, a spokeswoman for the county Department of Corrections, referred questions to County Executive Jack B. Johnson's office. James Keary, a spokesman for Johnson, said officials would not comment on a personnel matter.

At Easley's instruction, Merchant did not speak for the record. But Easley said: "It was wrong. It was terrible what Fairfax County did. It is horrible what Prince George's did. It's frightening that they can make these kinds of allegations. Without the video, she would have faced a year in jail."

The case started Feb. 8, Sylvester said, when a motorist on the Capital Beltway flagged down a state trooper and alleged that a black Mercedes had zoomed up behind him, turned on flashing blue lights inside the car's grill, forcing him off the road and then sped away. The driver jotted down the car's license plate number. Virginia State Police handed the case over to Fairfax police as a possible instance of police impersonation.

Officer Robert M. Bauer took up the investigation. He found that the license number was registered to Merchant and called her. The next night, Feb. 9, Merchant and her husband drove to a 7-Eleven in Annandale to meet Bauer and two other officers.

In court, Bauer testified that he searched Merchant's 2008 Mercedes S sedan and found neither blue lights nor an activation switch.

The encounter was captured on videotape. Bauer and Merchant are mostly out of view in the video, but their conversation was recorded fairly clearly.

On the tape, Merchant and her husband demand that the officer tell them who made the accusations. "I just need to know who filed charges so we can file charges," Merchant tells the officer. "It's very embarrassing. I'm a public safety person." She specifies that she is a deputy director of corrections in Prince George's.

Merchant becomes upset with Bauer, who declines her request, and demands to speak to his supervisor. When a lieutenant arrives, Bauer tells him, incorrectly: "She claims she's a deputy commissioner of police and fire. I looked her up; I can't find her. She pulls out a badge."