At first, the photo application revealed only a smoky room and an empty chair, Mr. Jackson said, but then a man sat down. Ms. Duplaga, again using remote technology, typed in the command to snap a photo. “When you take a picture with that computer, it shows a countdown, and when it does, this guy figures out what’s going on,” Mr. Jackson said. “It all clicks for him, and he puts his hand up to cover the lens, but it was too late. She had already taken the picture.”

Had the suspect been a complete stranger, the photographic evidence would have been a “great lead,” but not the decisive clue, Mr. Jackson said. He said that when Ms. Duplaga described the tattooed subject of the picture to one of her roommates, the roommate replied: “Oh, I know exactly who that is  it’s Ian,” referring to Mr. Frias.

Mr. Frias and Mr. Shahikian, it turns out, had been among the guests at a party at the apartment weeks before, and were friends of friends of the victims, as Mr. Jackson put it. Ms. Duplaga was able to retrieve a photograph of Mr. Shahikian from the laptop as well, but Mr. Jackson was not aware of the circumstances. Mr. Jackson said that Mr. Frias and Mr. Shahikian were arrested last year on a felony marijuana possession charge, but are not career criminals. The disposition of their cases was not known.

Mr. Jackson said that Ms. Duplaga did not wish to be interviewed. On Friday, no one answered the door at the beige colonial-style house on Ridgeview Avenue where she rents an apartment diagonally opposite a church. The neighborhood is on the edge of the city’s downtown; in recent years, many of the prewar homes have been renovated.

An Apple spokesman declined to comment on the case or on the phenomenon of tracking criminals through computer software.

“Back to My Mac” is part of an online service that costs $99 per year and allows users to gain access to their personal computer from any connected Macintosh computer with the operating system Leopard. The software that Ms. Duplaga used to take a picture of the thief, called PhotoBooth, is standard on all newer Apple laptop models, perhaps an unintended new frontier in crime-fighting.

“It’s certainly a great use of what was probably meant as a business product,” Mr. Jackson said. “But if she had taken the picture and didn’t know who he was, we wouldn’t be in the same place as far as the investigation goes.”