A Montgomery County jury yesterday found James Arthur Calhoun guilty of murdering a county police officer and a security guard during a March robbery at the W. Bell and Co. discount store in White Oak.

Calhoun, a 27-year-old escapee from Lorton reformatory, was convicted on nine charges in connection with the March 27 robbery, including a charge of attempted murder of a Bell Co. assistant manager critically wounded during the incident.

The jury of eight women and four men deliberated for about 10 hours before reaching its decision yesterday afternoon. The same jury now must decide whether to impose the death penalty on Calhoun for the killing of police officer Phillip C. Metz, 33.

Prosecutors are seeking Calhoun's execution on grounds that Metz's murder was premeditated and that Calhoun was supposed to be serving a life sentence for armed robbery at the time of the slaying.

This will be the first time that a jury will decide whether to put a convicted murderer to death since the Maryland General Assembly voted two years ago to give that sentencing power to juries instead of judges.

Prosecutors believe that Calhoun's codefendant, Curtis Wayne Monroe, 23, fired the gun that killed security guard David Myers, 30, during the robbery. Monroe is scheduled to go on trial Nov. 30. Calhoun was convicted of Myers' murder under a Maryland law that allows anyone convicted of taking part in a felony during which a murder is committed to be found guilty of murder even if that person did not actually pull the trigger.

Myers' parents, who attended most of the two-week trial, said they were pleased with the verdict and now want to see Calhoun put to death. "I feel that Dave is looking down and saying 'great,' " said Eileen Myers, the victim's mother.

Bell Co. assistant manager Douglas T. Cummins Jr., a key witness in the trial, testified that he, Metz and Myers were investigating a burglar alarm signal at the Bell store on New Hampshire Avenue in White Oak in the early morning hours of March 27.

Cummins said when he and his companions approached a back room where company safes are kept, a masked gunman grabbed Metz and shot him while another assailant jumped out and opened fire on Myers and Cummins.

Metz was shot in the head by a pistol pressed tightly against his temple, and Myers was shot in the heart. Cummins, wounded in the chest, then was forced to open two safes and left handcuffed to a file cabinet while the gunmen fled.

Cummins testified that, dragging a file drawer with him, he managed to struggle to an outside telephone to call police before losing consciousness. He identified Calhoun as the person who shot Metz even though the assailant was disguised with a stocking mask.

Prosecutors based their case essentially on the testimony of three witnesses -- Cummins, Herbert Smallwood, identified as the lookout man during the Bell breakin, and Calhoun's girlfriend, Audrey Neeley.

Smallwood agreed to testify against Calhoun in return for an agreement that he will not be prosecuted. He told the jury that the plan was for Calhoun and Monroe to wait inside the store until someone came to open it and then force that person to hand over the money in the safe.