The motive for the Monday murder of a woman and suicide of a man in McLean remained a mystery yesterday as friends of Christopher J. Thomas, 42, the unemployed engineer who shot himself and a former colleague, said he was despondent about being out of work since February.

But a spokeswoman for Thomas' former employe, TRW Inc., said Thomas had recently accepted a similar job with the company and was scheduled to report for work in its California office in June.

"He was at my house two weeks ago. He seemed normal, but somewhat despondent about not working. He didn't like the offers he was getting," said Pam Moritz, a friend and former coworker of both Thomas and Lucy A. Pirtle, 23, the systems analyst whom Thomas fatally shot in the parking lot of Commons Shopping Center off Rte. 123 after they had eaten lunch together.

Thomas, who had a PhD in mechanical engineering from Catholic University, was a native of Sri Lanka and had been in this country, along with his mother and three sisters, since 1968.

Thomas and Pirtle had worked together at TRW's Tysons Corner offices before Pirtle, who lived with five women in a house in Northwest Washington, was laid off in 1981. Described by friends as petite, vivacious and "very talented," Pirtle quickly found a job with a nearby computer firm. When Thomas was laid off this year, Pirtle gave a copy of Thomas' resume to her new employer, said Dr. Robert Shotwell, Pirtle's former supervisor.

Friends and colleagues described the relationship between Thomas and Pirtle as "businesslike" and "casual," saying the two saw each other infrequently after Pirtle was laid off. They expressed surprise at the report of an eyewitness who said the pair had left the restaurant hand-in-hand only minutes before the shooting.

"I had never heard his name," Pirtle's mother, Lucy, said yesterday. "He wasn't that important to her."

Thomas was described by a former coworker as "polite, quiet, not the type to even own a gun, let alone use it." He was unmarried and lived alone in a Falls Church apartment. Colleagues said he always went unaccompanied to company outings and social events.

Fairfax County police said yesterday that Thomas bought the .32-caliber handgun he used in the shooting from a Northern Virginia gun shop about a year ago.

Witnesses said Pirtle and Thomas ate a quiet lunch together and then walked through the parking lot to Pirtle's car. Once there, Pirtle "pulled away" from Thomas "as though she was irritated," a witness said. Thomas then shot Pirtle in the head and then shot himself.