Seven people have been arrested in connection with a plot to blow up Chicago's Sears Tower and other targets, the FBI said.
The arrests were made after a warehouse in Miami's Liberty City area was raided by agents last night, a spokesman for the agency said.
The alleged plotters were mainly Americans with no apparent ties to al Qaida or other foreign terrorist organisations.
"There is no imminent threat to Miami or any other area because of these operations," FBI's Washington spokesman Richard Kolko said.
Residents living near the warehouse said the men taken into custody described themselves as Muslims and had tried to recruit young people to join their group.
The residents said FBI agents spent several hours in the area showing photos of the suspects and seeking information. They said the men had lived in the area about a year.
The men slept in the warehouse, local resident Tashawn Rose, 29, said.
"They would come out late at night and exercise," she said. "It seemed like a military boot camp that they were working on there. They would come out and stand guard."
She talked to one of the men about a month ago, adding: "They seemed brainwashed. They said they had given their lives to Allah."
Ms Rose said the men tried to recruit her younger brother and nephew for a karate class.
The 110-storey Sears Tower is the nation's tallest building, at 1,450ft. Its skydeck was closed for about a month and a half after the September 11 attacks.
Several terrorism investigations have had south Florida links. A number of the 9/11 hijackers lived and trained in the area, including ringleader Mohamed Atta, and several plots by Cuban-Americans against Fidel Castro's government have been based in Miami.
Jose Padilla, a former resident once accused of plotting to detonate a radioactive bomb in the US, is charged in Miami with being part of a support cell for Islamic extremists. Padilla's trial is set for this autumn.
A man calling himself Brother Corey and claiming to be a member of the group told CNN that the individuals who worship at the building call themselves the "Seas of David."
He dismissed any suggestion that the men were contemplating violence. "We are peaceful," he said.
He added that the group studies the Bible and has "soldiers" in Chicago, but is not a terrorist organisation.
FBI Director Robert Mueller, questioned about the case during an appearance on CNN's Larry King Live, said he could not offer many details because "it's an ongoing operation."
"We are conducting a number of arrests and searches" in Miami, Mr Mueller said, which were expected to be concluded this morning.
Local resident Benjamin Williams, 17, said the group sometimes had young children with them. At times, he added, the men "would cover their faces. Sometimes they would wear things on their heads, like turbans."
Xavier Smith, who attends the nearby United Christian Outreach, said the men would often come by the church and ask for water.
"They were very private," Mr Smith said. "They spoke with like an accent, sort of a Jamaican accent."
A huge crowd - up to 250,000 people - was expected in the city today for a parade to honour the NBA champions Miami Heat. Security measures consistent with such an event were in place, city officials said, and the raids were not expected to affect it.Reuse content