Pipe bombs accompanied by anti-government propaganda exploded Friday in six mailboxes in rural parts of Illinois and Iowa, injuring six people in an attack authorities called domestic terrorism.

Two other bombs were found but did not detonate, and a note found with them said more "attention getters" were on the way. It was signed "someone who cares."

Authorities did not immediately announce any suspects. The Postal Service suspended deliveries through Saturday in the agricultural region that straddles the Mississippi River, and urged residents not to remove any devices they might find in their mailboxes.

"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."

In all, eight devices were found and six exploded. The bombs were not sent by mail but were instead placed in the mailboxes and set to detonate when the boxes were opened, investigators said.

John Peterson of the ATF said late Friday that the investigation was progressing well and officials from an FBI multi-agency terrorism task force and the U.S. attorney's office in Iowa's northern district had a number of leads.

"We are working with our profilers," he said.

Postal Service Inspector Linda Jensen said consistencies in placement suggested the bombs were linked, but that did not mean just one person was involved.

Peterson said it appears one person could drive to all the communities in a night, and it was safest to assume more devices have not been found.

"If you see anything unusual, wire, tape, string, anything of that nature, contact your local law enforcement. At the very least, be very careful," he said.

None of the injuries was considered life threatening, but Carroll County, Ill., Sheriff Rod Herrick warned residents against opening their mailboxes. "Don't touch your mailbox until further notice," he said.

Marjorie Zuidema said she and her husband, Robert, heard something that sounded "like a bird hitting a window, but a lot louder," when her mailbox in rural Morrison, Ill., exploded Friday afternoon.

By the time she discovered what had happened, "the police and FBI, postal inspectors, just about everybody" was at the end of her driveway.

Mail carrier Marilyn Dolieslager's face and left arm were injured and part of her thumb was blown off in the blast, according to her daughter, Jodi Camper.

Judy Temple, a neighbor of Zuidema, said she looked out her living room window just Dolieslager was pulling up.

"As soon as I saw her lean over in the mailbox, all of a sudden, 'Boom!' and everything when straight up in the air like a mushroom," Temple said. "I yelled for my husband, 'Something's wrong with Marilyn!"'

Temple and her husband called 911 and helped Dolieslager to their yard.

Also injured were three postal workers in Mount Carroll and Elizabeth, Ill., and Asbury, and two postal customers in Anamosa and Tipton. Only the Tipton resident remained in the hospital late Friday, officials said.

Other devices were found in Farley and a farm outside Davenport in Iowa.

Steve Ertmer was delivering mail near the town of Elizabeth when a pipe bomb detonated around 2:30 p.m. He suffered cuts to his hands, which required stitches, and his vehicle's door was damaged. But he said he probably was saved from more serious injuries because he was driving a four-door Blazer and sitting up high.

"I'm OK. I got lucky," he said Friday night. "I thank God it was me who opened it up and not my customer standing in front of it."

In Asbury, a bomb went off when a letter carrier opened a mailbox from the passenger side of a vehicle, leaving a small hole in the door, witnesses said. The carrier had surgery to remove shrapnel from his arm and suffered damage to his hearing, said Ken Runde, chief deputy in Dubuque County.

Runde said local law enforcement agencies could not check every mailbox in the area for a possible bomb.

"We probably have 30,000 mailboxes," he said. "If they notice something suspicious or foreign, then we will respond."

Postal Service vice president Azeezaly Jaffer said the bombs were accompanied by a typewritten 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. Jensen described the devices as three-quarter inch steel pipes with a 9-volt battery attached. Accompanying the bomb was a clear plastic bag containing the note.

Postal Service spokeswoman Joleen Baxa said mail delivery was suspended through Saturday in all rural areas east of Cedar Rapids and in northern Illinois.

Steve Rangel, a bartender near Morrison, said the quiet town is surrounded by farmland, which made Friday's explosions that much more surprising.

"I think everyone is angry that anybody would do something so ignorant," Rangel said. "It's got to be a sick individual. The whole thing is really sad."