The murder trial of James Arthur Calhoun, a Prince George's County man accused of murdering a Montgomery County police officer and a security guard in a W. Bell & Co. store last March, began yesterday in Rockville with defense lawyers attempting to cast doubts on the prosecution's key witness.

Prosecutor Barry Hamilton told the jury yesterday he will present testimony against Calhoun from Herbert Smallwood, who he said served as a lookout outside the store during the March 27 robbery and shooting, and drove one of the getaway cars.

Officer Philip C. Metz and security guard David Myers were both shot to death inside the store in White Oak early that morning. Douglas T. Cummins Jr., the store's assistant manager, was shot and left handcuffed to a file drawer after opening two safes for two assailants, who escaped with about $6,000 from one of the safes, Hamilton said yesterday.

Calhoun is charged with two first degree murder counts, one attempted murder count, robbery, and four related handgun charges. Prosecutors are seeking the death penalty.

Smallwood agreed to testify against Calhoun and codefendant Curtis Wayne Monroe in exchange for a guilty plea to lesser charges.

"Herbert Smallwood was the wheelman and the getaway driver," Hamilton told the jury in opening remarks. "From the point of view of one of the accomplices, you'll learn how it the crime was planned and how it was executed."

James Cromwell, one of Calhoun's two court-appointed attorneys, urged the eight women and four men on the jury to examine Smallwood's credibility "very, very carefully."

Cromwell said that in Smallwood's pretrial interviews with police, "he's told a variety of stories -- some this way, some another way -- a whole panoply of stories . . . . We ask you to consider the kind of person he is. He is a major cocaine dealer in the Washington area."

Cromwell said Smallwood himself will testify how he once earned "$2,500-a-week in drug trafficking."

"He made a deal, and he's permitted to plead guilty to a robbery charge in exchange for his testimony that 'I didn't do it, the defendent Calhoun did'," Cromwell said. "That is the man you have to believe."

Under Maryland law, the testimony of any so-called accomplice witness is not enough to convict a person unless it is corroborated by other supporting testimony or evidence. To corroborate Smallwood's version of events, prosecutors are relying on testimony from Calhoun's girlfriend, Audrey Neely, who is expected to tell the jury that Calhoun, who was unemployed, had an unusual amount of money days after the W. Bell murders and used the cash to take her on a shopping spree.

Yesterday's opening arguments came after lawyers concluded 3 1/2 days of jury selection, during which potential jurors were individually probed as to their feelings on the death penalty.