Nebraska mailboxes may be the latest target of a domestic terrorism campaign that has already injured six Midwesterners with pipe bombs.

"For the individuals or individuals who may have been responsible for this and who may be listening: You have gotten our attention," Weysan Dun, assistant special agent-in-charge of the FBI's Omaha office, said. "We are not certain we understand your message. We would like to hear from you. You do not need to send any more of these devices."

Six pipe bombs, none of which exploded, were discovered over the weekend in mailboxes in Nebraska, with at least one identical to bombs found Friday in eastern Iowa and Northwestern Illinois. The Nebraska bombs were also accompanied by letters, FBI Special Agent Jim Bogner said. Five were found in rural mailboxes on Saturday, a sixth in a residential neighborhood on Sunday, bringing the total over the weekend to 14.

Four postal workers and two residents in Illinois and Iowa were injured in the explosions, including one woman who remained hospitalized in fair condition Saturday.

Federal officials had described the earlier bombings as an act of domestic terrorism and said anti-government propaganda and notes warning of more "attention getters" were found nearby. Throughout the Midwest, the mail once again became an object of fear, bringing up echoes of the fright caused by the post-Sept. 11 anthrax mailings.

After Saturday's discoveries, postal inspectors in Iowa and Illinois curtailed inspections planned for thousands of mailboxes. Mail was suspended in the area and officials did not say how soon it would resume.

"We're using all of our resources for investigation and apprehension of whoever is responsible," said Rick Bowdren, inspector-in-charge of the Midwest division of the U.S. Postal Inspection Service.

He urged people across the Midwest to use caution in opening their mailboxes and said anyone who sees tape, wire or anything unusual around a mailbox should report it their local post office.

"We are asking postal patrons to keep their mailboxes open. We would recommend they tape it open," Bowdren said. "That way the carrier making a delivery can look in and patrons can look in and that anxiety factor will be alleviated."

The devices found Saturday, near the central Nebraska towns of Ohiowa, Columbus, Dannebrog, Davenport and Scotia, had been placed in the mailboxes, not sent through the mail, Dun said. He said four were found by mail carriers and one was discovered by a resident.

Gorlyn Nun said he wasn't aware of the explosions in Iowa and Illinois when he walked down his gravel driveway Saturday morning and opened his mailbox to find a pipe inside with a battery attached to it.

"I opened it up, my mail was there and there was a clear Ziploc bag in there. I could see it was a pipe and it had a 9-volt battery in there," he said.

The 59-year-old carpenter said he took his mail out and left the device behind, then called the local sheriff. The state patrol later arrived and detonated the pipe bomb.

Earlier Saturday, Bogner said authorities had some leads on who may have planted pipe bombs in at least eight rural mailboxes in eastern Iowa and northwestern Illinois on Friday, but they didn't know if one person or several people were responsible.

The note that had been left with the pipe bombs said more "could be delivered to various locations around the country," and postal officials in Washington on Friday had advised mail carriers across the country to be cautious.

Postal officials said the bombs that were found Friday were accompanied by typewritten notes in clear plastic bags 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."

It was signed, "Someone Who Cares."

Officials described the bombs as three-quarter-inch steel pipes attached to a 9-volt battery, which appeared to be triggered by being touched or moved.

Jon Petersen, a special agent with the Bureau of Alcohol, Tobacco and Firearms, said some of the bombs went off when the mailbox was opened and others went off when they were moved.

A map of the bombs found Friday forms a jagged circle straddling the Mississippi River and covering part of eastern Iowa and the northwest corner of Illinois.

Saturday's pipe bombs were found about 350 miles west of there, not far from Interstate 80, which runs through both regions.

In Illinois' Carroll County, Sheriff Rod Herrick spent Saturday morning opening mailboxes for worried residents. He fastened a clamp to the mailbox handle, tied fishing line to the clamp, then stepped behind his car and pulled on the line.

"It's no high-tech thing. I'm not a bomb expert," Herrick said. "But I need to do something to keep the calm here."

Postal officials were working with the Iowa state crime lab to devise a gadget similar to a fishing pole that would allow inspectors to open a mailbox without having to get close to it, said Ron Jensen, a postal inspector from Des Moines, Iowa.

But residents were still worried.

"You might find a beer can in a mailbox every once in a while around here, but not a bomb. Somebody obviously is screwed up in the head," said Cathy Meyer, an Ohiowa resident and former postmaster in the area. "This obviously is very, very troubling that someone would do this."

The Associated Press contributed to this report.