Khai Kong was manning the cash register in Olney's 7-Eleven store at about midnight Saturday when a darkhaired mustachioed man dashed in and demanded the telephone number for an ambulnce.

"He screamed at me when I didn't know it," Kong said yesterday. "Then he asked for change, and when he held out his hand, it was covered with blood."

It was shortly afterward, Montgomery County police said, that Phillip Lee Jr. called authorities from a phone booth outside the small grocery to report that his 64-year-old cancerstricken mother, Mary Elizabeth Lee, who lived a block away had been shot to death.

Police said they arrived at Lee's white wooden home on Hillcrest Drive at 12:40 a.m. to find Mars. Lee lying in bed with a bullet wound in the left side of her body. Phillip Lee, who was waiting there, was arrested and charged with murder, police said.

Police said yesterday they were questioning the 36-year-old Lee and were still unsure about the motives or causes behind the shooting. Neighbors and friends of the family said they were equally puzzled by the death of Mary Lee, whom they described as a hard-working, gentle woman who lived alone and had given up her job at Montgomery County Hospital only after she was weakened by cancer and the loss of one of her lungs.

"Both Mrs. Lee and her son were fine, they were laughing and talking together yesterday afternoon," said Hattie Sibley, who visited Mrs. Lee about eight hours before the shooting.

Phillip Lee, Sibley said, told his mother at about 4 p.m. as the three were sitting together that he was going out to "play music" with a band. "He seemed fine, not upset or anything," Sibley said. "I can't understand what happened."

According to friends of the Lee family, Phillip had moved from job to job during recent years, hampered by eye problems that had left him partly blind. He had married and fathered a child, friends said, but had separated from his wife.

Mary Lee, neighbors said, had been friendly but retiring during the eight years she lived on Hillcrest Avenue. Since her retirement a year ago, she had devoted herself to helping an elderly neighbor with a heart condition. "She was just a nice old lady," said neighbor J. O. Harvey, "Who never bothered anyone."