A Prince George's County Circuit Court jury yesterday sentenced Reuben Jackson Jr. to die in the gas chamber for the murder, kidnaping and robbery of Joanne Grossnickle. It was the first jury-imposed death sentence in the county since 1978, when capital punishment was reinstated in Maryland.

Grossnickle's body was found on Sept. 9, 1984, wrapped in a pink blanket in a grassy area in Cottage City six days after she was reported missing. Jackson, 40, of 3459 Minnesota Ave. SE, was arrested the next day by District of Columbia police who spotted him driving Grossnickle's car, which contained several items missing from her Silver Spring apartment.

Grossnickle was 22, a native of Frederick County, Md., and a recent graduate of Towson State College. She had just started working as a Capitol Hill lobbyist for the Church of the Brethren.

The evidence presented in the case against Jackson was mostly circumstantial. Police found his fingerprint on a letter Grossnickle's parents had left in her apartment, and some items missing from her apartment at his home. Jackson said he had gotten the keys to Grossnickle's car and apartment from a coworker, a man police never found.

On Tuesday afternoon, Jackson stood before the jury -- the same seven women and five men who had convicted him in July -- and asked them to spare his life.

"I am a man of peace. I don't go around hurting people, and I never have," Jackson told the jury. "I respect your verdict, but I don't agree with it because I didn't kill Miss Grossnickle. The Lord knows that I didn't do anything wrong."

He acknowledged his criminal past, four convictions, which Assistant State's Attorney Bond Rhue urged the jury to consider. Since 1963 Jackson had pleaded guilty or been convicted of two robberies, an escape and a theft, Rhue said during closing arguments.

Jackson's attorneys called more than 25 character witnesses to testify on behalf of their client. They described Jackson as a church-going man who studied the Bible and sang in the church choir. Reuben Jackson Sr. described his son as a man whose body bears the scars of a childhood with an abusive mother.

After the sentence was read, the jurors quickly left the courthouse, most of them declining to comment. One juror, who did not want to be identified, said that three or four members of the panel initially held out for a sentence of life in prison. But the juror said the "brutality of the murder" eventually led to the unanimous verdict.

"You can't feel good about sentencing someone to death," the juror said. "I don't think I'll ever feel good about it. It tests your fortitude."

Many of Jackson's relatives and a number of Grossnickle's survivors were present, but members of both families declined to comment.

Roberta Roper, the mother of Stephanie Ann Roper, a Frostburg State College student who was slain three years ago, sat through the hearing. After the verdict, she said she was "shocked and stunned" at the sentence. Roper, cofounder of a committee that has lobbied the state legislature for stiffer penalities for persons convicted of violent crimes, opposes the death penalty but views it as necessary unless life sentences are fully served.

"They [the jury] know the only way Jackson would not be free to murder again was to take his life," Roper said. "But why should we have to do that?"

Jackson's case, like all death penalty cases in Maryland, is automatically appealed to the Maryland Court of Special Appeals.

Twenty-three death sentences have been handed down in Maryland since 1980, according to Michael Millemann, a University of Maryland professor who keeps statistics on capital punishment cases. Nineteen people, including two women, are now on death row, he said. The last execution in the state was in 1964.