The U.S. Attorney General’s office arrested John P. Tomkins, a 42-year-old machinist from Dubuque, Iowa, on Wednesday, on suspicions he is the culprit who sent dud pipe bombs to the offices of American Century and Janus Capital with ominous messages signed “The Bishop,” the Associated Press reports. Tomkins, who was once a part-time U.S. Postal Service letter carrier, was on his way to work at a manufacturing facility. Federal agents began searching his home and a storage facility on Wednesday, and he was scheduled to appear in court in the afternoon.

Tomkins, who authorities believe intended to drive up stock prices, was charged with one count of mailing a threatening communication with intent to extort and one count of possession of an unregistered explosive device. The first charge carries up to 20 years in prison, the second, 10 years. Tomkins also faces a $500,000 fine.

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