At least four suspected pipe bombs were found Saturday in mailboxes in the midwest state of Nebraska, one day after a series of small mailbox bombs injured six people in the nearby states of Illinois and Iowa. Officials say they have some leads in their effort to find those people behind Friday's incidents.

The devices found Saturday in Nebraska were found in the southern part of the state, a few hundred kilometers from the communities where pipe bombs injured six people on Friday. A spokesman for Nebraska's governor said officials were checking out several reports of possible explosive devices.

Meanwhile in eastern Iowa and northwestern Illinois, mail delivery was suspended Saturday in communities within roughly 250 kilometers of Davenport, Iowa. That's the region where eight pipe bombs were found in rural, residential mailboxes on Friday. Seven of them went off, injuring six people. None of the injuries were serious.

Federal officials say the devices were not sent through the mail, but had been placed in the mailboxes. Many rural mailboxes are located by the roadside, where someone could easily put something in them unnoticed late at night. FBI special agent Jim Bogner is urging whoever is responsible to come forward. "He has, apparently, some grievances," he said. "We are trying to understand that. We are trying to ask him to reach out. We think that is a far better option to exercise than planting exploding devices and injuring innocent people who have nothing to do with this grievance."

Each of the devices found Friday was accompanied by a note described as "anti-government" in nature. The note complained about government's intrusion into people's lives, and promised that more "attention getters" would be coming in the future.

Postal authorities say they will inspect all 11,000 mailboxes in the area of Friday's incidents, to make sure there are no more unexploded pipe bombs that could hurt someone. The sheriff of one Illinois county spent Saturday morning checking the mailboxes of worried residents.