A note found with the bombs said "more 'attention getters' are on the way." It was signed "Someone Who Cares." Authorities did not immediately name any suspects.
Eight devices were found and five detonated. Postal Service Inspector Linda Jensen said consistencies in the placement of the devices suggested the bombings were linked, but that does not mean just one person was involved.
"We are reviewing this as a domestic terrorism incident," said James Bogner, an FBI spokesman. "We don't know if all the devices have been found or there are devices remaining ... We probably won't know for a while."
Jensen said bombs were found at Morrison, Mount Carroll and Elizabeth in Illinois, and at Asbury, Farley, Tipton, Anamosa and rural Scott County in Iowa. The communities form a rough triangle straddling the Mississippi River. None of the mailboxes are more than about 70 miles apart.
The bombs were set to explode when the mailboxes were opened, investigators said.
None of the injuries were considered life threatening, but Carroll County, Ill., Sheriff Rod Herrick warned residents against opening their mailboxes.
"I don't want kids getting off the bus and opening the mailbox, or people coming home from work and opening their mailbox," he said. "Don't touch your mailbox until further notice."
Postal Service vice president Azeezaly Jaffer said the bombs were accompanied by a note that began: "Mailboxes are exploding! Why, you ask?"
Then it said, in part:
"If the government controls what you want to do they control what you can do. ... I'm obtaining your attention in the only way I can. More info is on its way. More 'attention getters' are on the way."
The letter also said: "If I could, I would change only one person, unfortunately the resources are not accessible. It seems killing a single famous person would get the same media attention as killing numerous un-famous humans."
The bombs appeared to be triggered by being touched or moved and were attached to batteries, Jaffer said. He said the bombs were not sent through the mail but had been placed in the mailboxes.
Postal Service spokeswoman Joleen Baxa said mail delivery would be suspended through Saturday in all areas east of Cedar Rapids and in northern Illinois. The Davenport postmaster pulled his letter carriers off their routes.
"If you see a letter carrier, tell them you heard on the radio that they should report to their post office," Postmaster Dan Foley said.
One of the explosions happened in Sageville, just north of Dubuque. Witnesses said the bomb went off when a letter carrier opened a mailbox from the passenger side of a vehicle, leaving a small hole in the door and injuring the carrier's arm.
Donna Millwright, a letter carrier in Dubuque County for 22 years, stopped her vehicle along U.S. Highway 52 when she heard news of the explosion, waiting for orders on how to proceed.
"I don't want to get blown up," Millwright said. "I've seen plenty of things in mailboxes, but never a bomb."