A jury decided last night to spare the life of James A. Calhoun, 37, who was once condemned to death in Maryland's gas chamber for killing a Montgomery County police officer.

The Montgomery Circuit Court jury deliberated about three hours before returning a sentence of life in prison. Calhoun, whose 1981 death sentence in the case had been overturned by the courts, jumped up and hugged the three attorneys who represented him at his two-week sentencing retrial.

Earlier in the day, Calhoun told the jury that he was a changed man and pleaded for his life.

He was convicted of killing Officer Philip C. Metz during a March 27, 1981, robbery at the W. Bell & Co. store in Silver Spring. Metz and alarm system technician David Myers were shot to death in that robbery, while an assistant store manager was shot and left handcuffed to a file drawer.

Last night, jury members said they sentenced Calhoun to life because county prosecutors failed to prove he was the gunman. "The state just didn't have enough evidence," said jury forewoman Carol Ann Tantum, a systems analyst from Germantown. "They placed him at the scene, but no one proved he did it," she said.

Calhoun's death sentence, imposed by a jury in November 1981, was overturned after a 1988 U.S. Supreme Court ruling that found a written form used by jurors in capital punishment cases to be unconstitutional because it failed to give enough weight to mitigating circumstances.

The jury's only options yesterday were to sentence Calhoun to life or death. After closing arguments, Calhoun asked the jury not to send him back to Maryland's death row, where 12 other prisoners await execution.

"I am truly deeply sorry two innocent people lost their lives," said Calhoun, dressed in a gray suit, starched white shirt and burgundy tie. "I know I will spend the rest of my life in prison. I've come to grips with that. I hope you will open your hearts and look at me as a human being."

After the jury's verdict, Ruben directed that Calhoun's life sentence be served consecutively with prior sentences ranging from 15 to 50 years for variety of other charges including armed robbery, attempted murder and handgun violations.

Jurors said last night that they doubted the credibility of the prosecution's key witness, Herbert Smallwood, an accomplice who testified against Calhoun in exchange for a reduced sentence. Another defendant in the 1981 robbery and shooting, Wayne Curtis Monroe, was given a life sentence for killing Myers.

At Calhoun's retrial, defense attorneys tried to show that he is a different person from the "cold-blooded" killer portrayed by prosecutors.

Calhoun's mother testified her son was brutally beaten as a youngster with an electrical cord by his father. Several Maryland Penitentiary inmates testified Calhoun has been a stabilizing influence in the prison since his conversion to the Moslem religion. And Calhoun told the jury his life has a new focus: to prevent young people "from making the same mistakes I did."