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Petersen was a New York City police officer who worked in the R&D lab. When Tessa Mancuso's experimental new gun saved her life by shooting a robber by itself, she took it to Petersen so he could figure out how it did that. Petersen dismantled the gun and uncovered its artificial intelligence chip, whereupon the Seventh Doctor arrived and showed them that the chip contained James McIlveen's mind. (PROSE: Cat's Cradle: Warhead)