STAMFORD, Conn.  The letters that began arriving at a woman’s childhood home in Bridgeport, Conn., in May 2006 were as macabre as they were childish looking. One had a tombstone with her name on it and a drawing of a heart with a bite taken from it. Another had stick figures of grade-school children, a severed head and the warning “Prepare yourself.” The police traced the letters to an address in Minnesota, where they found tragedy waiting to happen.

Anthony M. Perone, a ninth-grade dropout who rarely left an upstairs bedroom in his parents’ house in Fairmont, Minn., had a knapsack full of ammunition and a recently purchased assault rifle with a scope when local and federal authorities arrived in June 2006. They said that Mr. Perone, now 21, appeared to have been preparing to take a trip to Connecticut, where he grew up, having packed a suitcase containing a machete, binding wire and a to-do list reminding him to “break in at night.”

His room, smelling of urine, was cluttered with hundreds of pages of drawings and notes in which he spoke of wanting to harm the woman, with whom he had attended grade school in Bridgeport, as well as her family and some of their third- and fourth-grade classmates.

Yet federal authorities said they might not have ever identified him had his mother  thinking she was being helpful  not written his name and return address on the letters he had given her to mail.