Remnants from anti-government notes found with the bombs warned of more "attention getters," and federal authorities described the apparently random bomb placements as an act of domestic terrorism.
Officials pleaded Sunday for whoever was responsible for leaving the pipe bombs in mailboxes from Nebraska to Illinois to contact authorities to make the intended message clear.
"You have gotten our attention. We are not certain we understand your message. We would like to hear from you. We are listening," Weysan Dun, assistant special agent-in-charge of the FBI's Omaha office, said Saturday.
"You do not need to send any more attention getters," he said.
"I hope whoever is responsible would respond," said Thayer County Sheriff David Lee, who's department received a call on a bomb found in a rural mailbox near Davenport on Saturday.
CBS News Correspondent Cynthia Bowers reports that authorities say they have promising leads, but may have partially destroyed their best clues - the rambling anti-government letters found along with several of the bombs.
The notes were damaged when the devices were neutralized.
"We're still trying to get this thing put together. We are aggressively investigating," said FBI spokesman Pete Sakaris in Omaha.
Among the six people injured when bombs exploded Friday in rural parts of eastern Iowa and northwestern Illinois, only a 61-year-old woman remained hospitalized Sunday. Doris Zimmerman, who lives near Anamosa, Iowa, was listed in fair condition.
The other two bombs found Friday in Iowa and the six found Saturday in Nebraska didn't go off, even though at least two of the devices had been picked up or moved by people reaching for their mail. Five were in rural roadside boxes; the sixth was in a mailbox in a residential neighborhood near Seward.
Gorlyn Nun said he saw a metal pipe, 9-volt battery and plastic bag in his mailbox near Ohiowa, but simply moved it out of the way. He later learned about the other pipe bombs and called authorities, who detonated the device.
"I think this is just kind of a random thing," Nun said. "I don't feel threatened or unsafe. We just happened to end up in this guy's path."
While authorities continued to search Sunday for the person or group responsible for the pipe bombs, Postal Service officials were determining whether mail delivery to roadside boxes in the areas would resume on Monday or remain suspended, spokesman Richard Watkins said.
Watkins said there were no warnings and nothing to indicate that whoever was planting the bombs had a grievance against the agency.
"If there is something on file, we'll find it, but that's part of the ongoing investigation," Watkins said.
Clint Van Zandt, a retired FBI profiler who leading the team credited with identifying the Unabomber, said the use of the term "attention getters," in the note may indicate the writer is older. Other aspects indicate the writer may have mental health problems, is probably a man and likely is working either alone or with a few close confidants, he said.
That the bombs were planted overnight indicates they likely were not targeted at rural residents but rather at the mail carriers, Van Zandt said.
"It's his belief we wouldn't go to the mailbox until the mailman got there," he said. "His target is either the post office itself or mail carriers as representatives of the government."
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.
The FBI and U.S. Postal Service both urged residents and mail carries to use caution when opening mailboxes and to report anything suspicious.
"We are asking postal patrons to keep their mailboxes open. We would recommend they tape it open," said Rick Bowdren, inspector-in-charge of the Midwest division of the U.S. Postal Inspection Service. "That way the carrier making a delivery can look in and patrons can look in and that anxiety factor will be alleviated."
Nebraska mail carrier Lyle Bartels said he was worried about returning to his largely rural route. Two of the pipe bombs were in his area.
"I'm just going to try to look the boxes over a little bit before I open them," said Bartels, of Ohiowa. "It's kind of scary."