A Prince George's County Circuit Court jury convicted Reuben Jackson Jr. yesterday of the murder last September of 22-year-old Joanne Nancy Grossnickle, a native of Frederick County who had just started a career as a lobbyist on Capitol Hill.

State prosecutors said they will seek a death penalty in the sentencing trial scheduled to begin before the same jury Sept. 17.

The panel took less than two hours yesterday to convict Jackson, 40, a resident of Southeast Washington, on charges of first-degree murder, kidnaping and robbery. Grossnickle, who disappeared last Labor Day after leaving her parents' house in rural Union Bridge, Md., to drive to her new apartment in Silver Spring, was found stabbed to death in Cottage City on Sept. 9, six days later.

On Sept. 8, police had arrested Jackson in the District after he was seen driving Grossnickle's 1984 Nissan. They said that belongings missing from the young woman's apartment were found in the car and at Jackson's apartment on Minnesota Avenue SE.

Jackson's left thumbprint was also found on a note Grossnickle's mother left in the Silver Spring apartment following the disappearance, according to Montgomery County police.

Joanne Grossnickle's parents, who were not present when the verdict was read, told reporters outside the courthouse later that they were satisfied with the verdict.

"Our thoughts are that justice has taken its course," Byron Grossnickle said.

"I suspect they the jurors knew their decisions when they walked out of the courtroom," Virginia Grossnickle said.

While Jackson's blank expression did not change when the jury's decision was read, his attorney, James Kenkel, said he himself was "emotionally devastated" by the verdict. He said he remains "personally, morally, and legally convinced that Jackson had nothing to do with these crimes."

Kenkel said that after sentencing he will appeal the verdict and the sentence.

Assistant State's Attorney Bond Rhue said that the "totality of the evidence" made the case a "clear and convincing" one, although he acknowledged that he was surprised by the quick delivery of the verdict.

Defense attorneys Kenkel and Patrice Lewis had maintained throughout the trial that evidence failed to show that Jackson ever kidnaped or murdered Joanne Grossnickle. During closing arguments yesterday, Kenkel said that Rhue had failed to provide jurors with "some big pieces" of a "puzzle" that would explain the circumstances of the woman's death.

After the verdict was delivered, Rhue said he thought the jury had been persuaded by the "overwhelming" amount of evidence presented. Several pieces of furniture, pictures, a television and stereo, and other possessions of Grossnickle that had been found in Jackson's apartment were displayed in the courtroom.

"We just laid out the facts . . . and we were very careful not to parade the evidence," the prosecutor said.

Following the verdict, Kenkel asked Judge Arthur Ahalt to allow a different jury to determine if Jackson should receive the death penalty, but that motion was denied.

Two of Jackson's relatives who were waiting outside the courtroom refused to comment on the verdict.

Joanne Grossnickle's family and coworkers became concerned when the woman did not show up for work the Tuesday following Labor Day. After notifying police that their daughter was missing, the Grossnickles went to her apartment at Georgia Avenue and Fidler Lane, where they immediately noticed that certain items were missing, they testified.

Virginia Grossnickle told jurors last week that she returned to the apartment on several occasions and each time noticed that more of her daughter's belongings were missing from the apartment. On the second trip to the apartment, Mrs. Grossnickle left her a note asking her daughter to contact the family or authorities.

Police said they subsequently found Jackson's fingerprint on the note.

Melva Jimerson, who worked with Joanne Grossnickle on the Hill, described her as "a quiet and lovely but a very strong person," and said yesterday that she was happy with the verdict.