The 30-year-old Malden man was allegedly telling her how to set up a web camera so that she could send him nude pictures of herself. As he was typing away, two Marshfield police detectives walked up to his desk yesterday at Lambda Research Corp., in Littleton.

They promptly arrested Thomas and charged him with 13 counts of attempting to disseminate harmful material to a minor and one count of attempting to pose a child in the nude. He was scheduled to be arraigned in Plymouth District Court today.

The arrest was the work of a year-old task force formed by Marshfield police and the Plymouth County Sheriff's Department. The task force, designed to fight computer crime, is known as HEAT, or the High-Tech Evidence Analysis Team.

Its core members are Marshfield Detective Steve Marcolini and an undercover agent with the sheriff's department. The team works with State Police, the Plymouth County District Attorney's office and other local police departments.

Formed in October 2004, team members spent their first year investigating identity theft and processing technological evidence such as computers seized from suspected drug dealers. Last month, the team moved into undercover work.

Believing that he was chatting with a 13-year-old, Thomas started using sexually provocative language with the undercover agent, instructing her about how to perform sexual acts, and telling her about his past sexual experiences, Marcolini said.

Thomas even mailed $60 in cash to the agent so that she could buy herself a web camera. The agent was chatting with Thomas yesterday afternoon when Marcolini and another detective arrested him.

Police confiscated Thomas' office computer. Marcolini said Lambda Research, Thomas' employer, was extremely cooperative yesterday and was in no way implicated in the investigation.

Marcolini said police were also searching Thomas' home last night, and would most likely seize his computer. Thomas is the second alleged sex offender to be arrested by HEAT.

Last month, police arrested Robert Demarco, 45, of Brockton, after HEAT's undercover agent got him to drive to a Marshfield Starbucks to meet her.

Demarco thought he was going to meet two girls, ages 14 and 15, police said. Police arrested him when he arrived at the coffee shop. He was charged with two counts of attempted statutory rape, two counts of enticing a minor into sexual intercourse, six counts of disseminating harmful material to a minor and resisting arrest. Those charges are pending.

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