Timeline of mailbox pipe bomb incidents:

Friday: Eight pipe bombs found in roadside mailboxes near Iowa towns of Eldridge, Farley, Asbury, Tipton and Anamosa and the Illinois towns of Mount Carroll, Morrison and Elizabeth. Six bombs explode, injuring four mail carriers and two residents.

Saturday: Six pipe bombs found in mailboxes near Nebraska towns of Dannebrog, Ohiowa, Davenport, Columbus, Scotia and Seward. None explode.

Sunday: Pipe bomb found in a mailbox near Albion, Neb.; the device doesn't explode.

Monday: Pipe bombs found in mailboxes near Hastings, Neb., and Salida, Colo.; none explodes. Device found in mailbox in Amarillo, Texas, described as similar to the others.

Tuesday: FBI issues all-points bulletin for 21-year-old college student, Luke J. Helder of Pine Island, Minn. Helder arrested outside Reno, Nev., and charged.