The man, dressed in a wig and dress and high heels, knocked on Novella Lee Harris's front door the night Beverly R. Mitchell disappeared.

Harris looked out from her Potomac Avenue row house in Southeast Washington and saw the person she knew as "Cookie." Parked behind him at the curb was Mitchell's car.

A Charles County Circuit Court jury yesterday decided Cookie was, in fact, Garrison Thomas. It convicted the District man of strangling and beating to death Mitchell, 26, of Alexandria, more than four years ago.

The jury deliberated nine hours over two days before finding Thomas, 44, guilty of second-degree murder; first-degree felony murder, which is a slaying that occurs during the commission of a felony; and robbery, for taking Mitchell's Mitsubishi Eclipse automobile.

Thomas could be sent to prison for life when Judge Richard J. Clark sentences him Aug. 12.

"I'm delirious. Victorious is the word," said the victim's mother, Marva Mitchell, of Camp Springs, minutes after the verdict was returned. "This is what we want."

Defense attorneys said they would appeal. Betty Stilt, an assistant public defender, said the case should have been dismissed because of the disappearance of crucial witnesses, including two who could have tied other people to the victim's car.

Mitchell was last seen alive about 10 p.m. on March 22, 1995, as she dropped off $10 with her uncle at his Southeast Washington home, where Thomas lived in a basement rental.

The next day a man driving off-road spotted Mitchell's body about three miles west of La Plata. Her skull was caved in. A fist-size rock and a stout log, both bloodstained, lay nearby. No evidence during the eight-day trial indicated how or why Mitchell's body ended up some 20 miles from Washington.

Jury foreman James R. Gilliam Jr. said Thomas was tied to the crime by testimony from Harris, the woman who opened her door to Cookie.

Harris testified that she and Cookie spent much of the next two days bingeing on crack cocaine. Cookie claimed to possess the victim's car but refused to use it, Harris testified.

She said Cookie took a taxi to and from the Wheeler Road area, where Thomas lived. Harris described Cookie as having acne; Thomas has blotchy skin. Cookie dressed as a woman; two other witnesses said they had seen Thomas wearing women's clothes.

"It was just too much coincidence," Gilliam said.

Defense attorneys argued that Harris was not to be trusted and cited a dearth of physical evidence linking Thomas to the crime.

"All the things she said just jibed," Gilliam said. "Even though we knew she was a crack addict . . . it was her descriptions that sort of nailed it in place for us."

CAPTION: Beverly R. Mitchell, 26, of Alexandria, was slain in 1995. A Charles County jury convicted Garrison Thomas yesterday of two counts of murder in her death.