A masked robber killed a bank guard with the guard's gun yesterday and then fled with an undisclosed amount of cash from the Seat Pleasant branch of Suburban Bank, according to Prince George's County police.

James E. Brown, 40, of 5070 Just St. NE, died about two hours after the 8:45 a.m. robbery, according to police.

Inside the family's home late yesterday, Brown's wife Mary, who is expecting the couple's third child, was sorting through family documents and declined to discuss the incident.

FBI Special Agent Joel M. Sanders said that an armed man wearing a ski mask entered the front door of the Suburban Bank at 5800 George Palmer Highway and aimed a gun at Brown.

Based on information from witnesses, Sanders said the suspect ran out of bank, and Brown chased him. The suspect then entered the bank a second time, through a side door. When Brown reached him, the two men scuffled, and the suspect fired at Brown with the guard's own gun.

According to police, the robber then vaulted over a teller's station, took an undisclosed amount of money and fled on foot. Last evening, FBI agents, who routinely investigate bank hold-ups, had made no arrests in the case.

Brown was rushed to Prince George's Hospital where he died a short time later.

Bank spokeswoman Dianna Bailey said that Brown had worked as a guard for the bank since 1979.

Later in the day police were told by witnesses that the holdup man had ridden a red motorcyle to the District line, parked it on Eastern Avenue, and walked about two blocks to the bank. After robbing the bank, police said, the man, carrying a black motorcycle helmet, ran back to his parking place only to discover that someone had just taken his motorcycle. Police said the suspect chased the person with the motorcyle, reclaimed it and rode away.

Yesterday, Brown's neighbors stood on the sidewalk near the place where Brown had been waxing his car just the night before. Lucille Doxie shook her head sadly and said, "It doesn't make any sense that thugs would come take his life like that."

According to Doxie and others, Brown was a quiet man who was very popular in the row-house community just off Sheriff Road. "Any time you had your car hood open, he was there to help," Doxie said.

A friend said Brown's wife received a call yesterday morning at her job at C&P Telephone, informing her of the shooting.

As the wife spoke quietly with friends and answered numerous phone calls, the security guard's stepfather, Ralph Brown, played ball outside with Brown's sons, Timothy, 10 and Terrance, 7. At one point the younger boy went back into the house and emerged again wearing his father's dark blue baseball cap.

Ralph Brown leaned against a car, watching the red-eyed boys and clenched his fist, remembering how James Brown enjoyed "rassling" with them in their tiny front yard. "Now they don't have anyone to do that with them," the older man said. "Those boys won't ever live this down."