Pipe bombs wound 5 in Iowa, Illinois

Warning: 'Do not touch the item or slam the box door'

An investigator takes photos of a crime scene where a bomb exploded after a rural mail carrier opened a mailbox north of Dubuque, Iowa, on Friday.

WASHINGTON (CNN) -- Five people were wounded Friday when pipe bombs placed in rural mailboxes in Illinois and Iowa exploded, prompting the U.S. Postal Service to call carriers back and suspend Saturday delivery service in the area.

"At this point, it appears as though the explosives devices were intended to detonate when they were removed from rural boxes, mailboxes," U.S. Postal Service Inspector Linda Jensen said.

The eight pipe bombs -- placed in primarily rural areas of the two states -- were made from three-quarter-inch pipes with a 9-volt battery attached. Postal officials said the bombs did not go through the U.S. mail and appeared to be placed in mailboxes randomly.

Each bomb was accompanied by a plastic bag containing a letter filled with anti-government statements and threats of more bombs. (Read the note)

CNN NewsPass VIDEO U.S. Postal Service Inspector Linda Jensen warns not to touch the devices or slam mailbox doors shut (May 3) Play video   EXTRA INFORMATION Mailbox bomb letter text

"We looked at the letter that had accompanied the devices and are viewing it as a domestic terrorism incident," said FBI Special Agent James Bogner. "There is indication that there may be other devices."

The letters were either the same or contained similar information, he added, and did not indicate how many others may exist -- but it did promise more "attention getters" are on the way.

The FBI, U.S. Postal Inspection Service and the bureau of Alcohol, Tobacco and Firearms have formed a joint task force with the U.S. Postal Service.

Explosive devices were found in areas near the Iowa-Illinois border, including Morrison, Mount Carroll and Elizabeth, Illinois, and at Asbury, Farley, Tipton, Anamosa and Scott County in Iowa.

No threats were phoned in about the devices to either postal officials or the FBI, officials said.

Jensen said it's unclear whether the culprit acted alone or with others in placing the devices.

"There are some consistencies among the placed devices that would indicate that there may be some effort that was related, but it's really early in the investigation at this point," she said.

One of the bombs detonated in Morrison, Illinois, as a rural mail carrier opened the mailbox to deliver mail, a Whiteside County Sheriff's dispatcher said. The mail carrier suffered lacerations to her arms and face.

A plastic bag containing a typed letter was found lying on the ground about 8 feet away from the mailbox, the dispatcher said.

Around 6:30 a.m. (7:30 a.m. EDT), two people in Scott County, Iowa, went outside to get the morning paper and saw wires around the door of their mailbox, said Capt. Mike Brown of the county sheriff's department.

The couple opened the mailbox and saw the bomb, which did not detonate, he said. The couple was not injured. Brown said the county bomb squad went to the site and removed the device.

A couple of hours later, the sheriff's office discovered the other cases in the region, at which point they began working with the FBI, ATF, U.S. Postal Inspection Service and state agencies.

Jensen said all carrier or postal customers should be very cautious when opening mailboxes and should take special care if wires, a pipe or a letter in a clear plastic bag are found.

"Please, please, do not touch the item or slam the box door," she said.

Customers who find suspicious items or notice someone putting things in mailboxes are urged to call the local police or the U.S. Postal Inspection Service command center in St. Louis at (314) 539-9310 and immediately clear the area around the mailbox.

-- CNN producer Terry Frieden contributed to this report.