An administrative assistant to Fairfax County's chief prosecutor has been arrested on a forgery charge after she helped erase someone's drunken-driving conviction from county computers, police said.

Cecilia A. Ruiz, 29, of Herndon was arrested Monday and charged with forging a public document, Fairfax County police said. A familiar face in the courthouse, she had worked in the prosecutor's office for "a couple of years," Commonwealth's Attorney Robert F. Horan Jr. said, and was one of the top administrators in the busy first-floor office.

"I couldn't believe it," Horan said yesterday. He said Chief Fairfax Circuit Court Judge Michael P. McWeeny had appointed the Prince William County commonwealth's attorney's office to prosecute the case.

Police said the incident occurred in the fall last year. Horan said the police made him aware of their investigation recently.

Horan said a Fairfax police officer's persistence led to the arrest. He said that the officer had arrested a man on a drunken-driving charge and that several months after the man was convicted, the officer saw him driving again. He said that the man denied the conviction but that the officer was sure of it.

"He didn't let it go," Horan said of the officer. He said the officer eventually found the man's conviction had been changed to failing to have proper county tags.

Horan declined to discuss the man's link to Ruiz. He said he would fire Ruiz. The investigation is continuing, police said.

Ruiz is the third courthouse employee arrested in the last six weeks. Last month, two women who worked at the public service counter of the Circuit Court were charged with grand larceny. Their cases are pending.