The illicit haul arrived each day by e-mail, the personal details of computer users tricked by an Internet thief: a victim’s name, credit card number, date of birth, Social Security number, mother’s maiden name.

One more Internet “phishing” scam was operating. But this time, private sleuths soon were hot on the electronic trail of a thief whose online alias indicated an affinity for the dark side. The case moved ahead in part because of an underground tipster and the thief’s penchant for repeatedly using the same two passwords: “syerwerz” and r00tm3.”