Authorities executed warrants over the weekend in Michigan's Lenawee and Washtenaw counties for activity that could be connected to an extremist militia organization.
At least two people were arrested in Ohio, officials said.
A third arrest made in northeastern Illinois on Sunday stemmed from a raid Saturday just over the border in northwestern Indiana, both part of a continuing investigation led by the FBI in Michigan, according to agents in Illinois.
Sandra Berchtold, spokeswoman for the FBI's Detroit office, would confirm only that "there were law enforcement activities in the Lenawee/Washtenaw County area."
A memo from Michigan State Police issued to law-enforcement agencies in Michigan said numerous federal search and arrest warrants were being executed by the FBI, state police and other agencies belonging to the Joint Terrorism Task Force.
Lee Miracle, coordinator of the Southeast Michigan Volunteer Militia, said none of that group's members had been arrested. "It had nothing to do with our group and nothing to do with the bulk of the militia groups here in the state," he said.
On its Web site, lenaweemilitia.com, the organization posted a notice yesterday saying it had "not been raided by the FBI. We do nothing illegal."
The posting included a link to a report in Ann Arbor's online newspaper that said targets of the raids were members of Hutaree, a Christian-oriented militia group.
Miracle said the Southeast Michigan Volunteer Militia had no ties to Hutaree.
Michael Lackomar, a spokesman for the militia, said one of his team leaders got a frantic phone call from members of Hutaree who said their property was being raided by the Bureau of Alcohol, Tobacco, Firearms and Explosives.
"They said they were under attack by the ATF and wanted a place to hide," Lackomar said. "My team leader said, 'No, thanks.'"
According to Hutaree's Web site, the organization believes that one day there will be an anti-Christ, and all Christians must train and prepare.
Dawud Walid, head of the Michigan branch of the Council on American-Islamic Relations, said he was told the Christian militia group might have been plotting against American Muslims.
In Ohio, FBI agents from Cleveland arrested two people in Sandusky and Huron on Saturday.
Scott Wilson, an FBI spokesman in Cleveland, said he could not comment on whether those two arrests were related to the ones in southeastern Michigan.
One of the raids occurred at Bayshore Estates, a trailer park in Sandusky, park manager Terry Mills said.