In what authorities call a case of domestic terrorism, another pipe bomb was found in a rural Nebraska mailbox Monday, the eighth in the state and the 16th found in the Midwest.

Authorities also said a pipe bomb was found Monday in a mailbox in the small mountain community of Salida, Colo., 100 miles southwest of Denver.

A resident found the device in a sandwich bag with a piece of folded paper and alerted authorities. It did not explode.

FBI spokeswoman Ann Atanasio said the pipe bomb is similar to those found in three Midwest states, but investigators were doing more work to determine if it fit the pattern or was a copycat crime.

Authorities did not immediately release other details, including whether the paper was a note similar to the anti-government letter found with the other devices.

Pipe bombs wounded six people in Illinois and Iowa on Friday, while two other bombs found in Iowa did not explode. Over the weekend, seven bombs were found in Nebraska, but they were detonated harmlessly by authorities.

None of the wounded suffered life-threatening injuries.

The latest Nebraska pipe bomb was found near Hastings in the mailbox of someone who had been away for the weekend, authorities said. It did not explode. There was no immediate word on it was accompanied by the same anti-government note found with the other devices.

"We have confirmed that an eighth bomb has been found, beyond that we don't have any information," Postal Service spokesman David Failor said.

There have been no arrests in the case.

The FBI said Monday that the first 15 bombs clearly come from the same source, but officials have not said whether they are searching for an individual or a group.

The latest bombs were found as hundreds of nervous letter carriers went back to work across the Midwest. Rural residents in at least four Midwest states and Colorado were asked to leave their mailboxes open or remove the mailbox door as a safety measure.

Jim Pelzer wore safety goggles and earplugs as he delivered mail in Tipton, Iowa, where one of the bombs exploded Friday. The protective gear was a gift from his wife.

"My feeling was when we had 9/11 and the anthrax scare, I was a little concerned about my job safety," Pelzer said. "But now I'm intimidated and scared."

Mail carrier Doris Fehlhafer, who was working outside Seward, added: "With the boxes open, you feel a lot safer."

Authorities were not surprised by the discovery of the latest bomb in Nebraska because of an apparent pattern by the person or persons planting the devices, said Mike Matuzek, a Postal Service district manager.

The bombs in Iowa and Illinois were found in locations that form a large, uneven ring about 70 miles in diameter. The Nebraska bomb sites form a large ring of about 90 miles across.

The areas are separated by about 350 miles. Salida is more than 400 miles from the Nebraska sites.

The FBI said the bombs and the notes were nearly identical.

FBI agent Larry Holmquist said the only differences in the bombs were slight variations in the detonating mechanisms. He refused to elaborate.

"There is no question that these were planted by the same person or persons," Holmquist said.

Officials described the Midwest bombs as three-quarter-inch steel pipes attached to 9-volt batteries, and said they appeared to be triggered by being touched or moved.

The typewritten note found with the bombs read, 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 FBI considers the attacks a case of domestic terrorism, and profiling experts have said whoever wrote the note is probably an older American man.

Dan Mihalko of the Postal Inspection Service in Washington said there is no indication that the Postal Service or its employees are the intended targets.

"When this guy is talking about the government, but it [the note] never gets into specifics about the government," he said. He said the Postal Service could be "just a convenient place of dropping things off."

Mail delivery, which was suspended Saturday, resumed Monday with added precautions. Homeowners with roadside delivery in Nebraska, Iowa and parts of Illinois and Wisconsin left their mailbox doors open or removed them.

Mail carriers were told that if a customer's mailbox was not open, they should bring the mail to the door.

Bob Temple of Morrison, Ill., said he cautiously opened his mailbox Sunday night to ease his carrier's fears. Temple's carrier was wounded when a pipe bomb blew up in her face while she was delivering mail to his next-door neighbor.

"I was pretty confident that the people that done it probably wouldn't be back this way, but it did kind of scare me," Temple said. "It was a relief when the door opened and nothing happened."

The Associated Press contributed to this report.