An off-duty District police sergeant shot and killed a Southeast youth who tried to rob him early yesterday, D.C. police said.

Sgt. Jimmy Flournoy shot Ernest Windfield Brooks, 17, a sophomore at Eastern High School, at 12:50 a.m. in the 4400 block of Southern Avenue SE, according to police.

Police spokesman Sgt. Joe Gentile said Brooks, of 1307 45th Place, approached Flournoy, pulled a pistol and announced a robbery outside a liquor store.

A struggle followed and the officer drew his service revolver and fired one shot, hitting the youth in the neck, Gentile said.

Brooks was taken to D.C. General Hospital, where he was pronounced dead at 1:50 a.m., Gentile said.

Brooks was the fourth person fatally wounded by area law enforcement officers since Dec. 19.

Flournoy, 35, who is assigned to the city's 5th Police District, is a 15-year veteran of the police force. He has been placed on administrative leave while the shooting is under investigation, a routine procedure, police said.

Brooks' mother, Rosanna Brooks, said a homicide detective who visited her said that police had recovered a toy gun at the scene of the shooting. Gentile said that police had found a gun and added, "I know nothing of a toy gun." The detective could not be reached for comment.

A police source said Flournoy was at the telephone when the incident occurred.

Three months ago Brooks graduated from the Second Genesis drug program after using illegal drugs such as PCP, according to his mother. He had spent a total of 16 months in drug rehabilitation programs in the last two years, his family said.

The youth left home a short time before the shooting to make a telephone call to his girlfriend of six months, Netosha Clark, his mother said. The Brooks family has no telephone in their home, and Ernest Brooks often used a pay telephone near the scene of the shooting, Rosanna Brooks said.

It is uncertain whether he reached his girlfriend by phone before the shooting. Clark said Brooks always called her just before he went to bed and they talked often of how he had improved his outlook in the drug program.

"He told me he had really learned a lot, and that he wished he'd never started taking drugs," Clark said recalling earlier conversations with Brooks.

At the Brooks home yesterday, the youth's Christmas gift to his mother, a picture of the Last Supper, was on the wall.

"He told me he loved me, and that I should take care of myself," Rosanna Clark said.

Brooks' mother said she did not believe the police officer's account that her son was attempting to rob him, because Brooks had stopped taking drugs and had no need for money.

Brooks had received a gift of $20 cash for Christmas, the day before, she said.

"I don't know exactly what happened, but it couldn't have happened the way they said it did," the mother said. "I don't plan to let them sweep it under the rug."

Police declined further comment on details of the shooting.