Omaha Neb. ? Rural carriers planned to deliver mail as scheduled on Monday despite the discovery of 14 mailbox pipe bombs across the Midwest in recent days, authorities said Sunday.

But postal officials warned customers that the doors of roadside mailboxes must be kept open. Affected are customers in Nebraska, Iowa and northwest Illinois.

“We are instructing our carriers not to deliver to any closed receptacles,” Mike Matuzek, U.S. Postal Service district manager for Nebraska and southwest Iowa, said Sunday.

Locked mailboxes, like those at apartment complexes and neighborhood delivery units, will have normal delivery, he said.

Matuzek called it a temporary precaution while the investigation continues. “It is deplorable,” he said. “It brings a lot of innocent people into the fray of this thing where they really don’t belong.”

An 18-year-old man was arrested in St. Paul on Sunday night after at least one of two suspicious devices found in mailboxes in Nebraska on Sunday was determined to be a prank, the U.S. Postal Service said.

Roger Humphries, a postal service spokesman, declined to describe the latest devices. The Postal Service doesn’t want to deal with any more hoaxes, he said.

Officials on Sunday renewed pleas that whoever planted the bombs contact them and make their grievances clear.

“I hope whoever is responsible would respond,” said Thayer County Sheriff David Lee, whose department received a call on one bomb found in a rural mailbox near Davenport on Saturday.

The FBI also was posting a letter on the Internet that had been found with some of the devices on its Web site – www.fbi.gov – and the postal service Web site at www.usps.com.

“The public is invited to view the letter, which may be recognizable to someone,” the FBI said in a news release.

Six people were injured by explosions in Illinois and Iowa on Friday. None of the six bombs found Saturday in rural areas of Nebraska went off. They were later detonated harmlessly by authorities.

An anti-government note found with the bombs warned of more “attention getters,” and federal authorities described the bombs as an act of domestic terrorism.

Among the six people injured Friday, only a 61-year-old woman remained hospitalized Sunday. Doris Zimmerman, who lives near Anamosa, Iowa, was listed in fair condition.

“We’re still trying to get this thing put together. We are aggressively investigating,” said FBI spokesman Pete Sakaris in Omaha.

Mail carrier Lyle Bartels of Ohiowa, Neb., said he’ll be cautious when he returns to his route. Two of the pipe bombs found Saturday were in his delivery area.

“I’m just going to try to look the boxes over a little bit before I open them,” Bartels said. “It’s kind of scary.”

In Illinois, Carroll County Sheriff Rod Herrick said Sunday that most residents seemed to have gotten over their shock, although some people were asking him to use fishing line to remotely open their mailboxes as a precaution.

Lee, the Thayer County sheriff, said the message conveyed by the bombs is that terrorism can happen anywhere.

“Thayer County is just a rural, farming community, and I think that’s just showing that these kind of acts are going to reach everyone – not just large metropolitan areas,” the sheriff said.

Lee’s request on Sunday that the bomber contact authorities followed a similar message from the FBI on Saturday.

“You have gotten our attention. We are not certain we understand your message. We would like to hear from you. We are listening,” said Weysan Dun, assistant special agent-in-charge of the FBI’s Omaha office. “You do not need to send any more attention getters.”

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

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

Two bombs found Friday in Iowa and the six found Saturday in Nebraska didn’t go off, even though at least two were picked up or moved by people reaching for their mail. Five of the Nebraska bombs were in rural roadside boxes; the sixth was in a mailbox in a residential development outside Seward.