Omaha, Neb. ? Five pipe bombs were found in rural Nebraska mailboxes Saturday, officials said, heightening worries among Midwesterners already on edge after similar bombs injured six people a day earlier in Iowa and Illinois.

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.

It wasn’t immediately clear whether similar notes were found with the pipe bombs discovered Saturday.

Postal Service Vice President Azeezaly Jaffer confirmed Saturday afternoon that five pipe bombs similar to those discovered Friday had been found in mailboxes near the central Nebraska towns of Scotia, Cairo, Columbus, Davenport and Ohiowa. He said none of those bombs had exploded.

Earlier Saturday, federal authorities announced they had some leads on who may have planted the pipe bombs in 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 left with the bombs had said more “could be delivered to various locations around the country,” and postal officials in Washington had advised mail carriers across America to be cautious.

In all, eight pipe bombs were found in Iowa and Illinois on Friday, said Thomas Ahearn, spokesman for the U.S. Bureau of Alcohol, Tobacco and Firearms office in Chicago. Four postal workers and two residents were injured, but none had life-threatening injuries. One woman remained hospitalized in fair condition Saturday.

FBI special agent Jim Bogner said Saturday that authorities had some promising leads, and he invited whoever was responsible to contact the FBI.

“We want to assure him he has our attention and we want to understand what the situation is, because apparently he has some grievances,” Bogner said. “He has our attention and we want to listen now.

“It’s a much better option to exercise than planting bombs and injuring people who have nothing to do with these grievances.”

Postal officials said the bombs 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.”

The note was signed, “Someone Who Cares.”

Authorities still were collecting evidence at the scenes on Saturday, said Jon Petersen, a special agent with the Bureau of Alcohol, Tobacco and Firearms.

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.