As part of the raids tied to the arrests, FBI agents swarmed a warehouse in Miami's Liberty City area, using a blowtorch to take off a metal door.
The official told The Associated Press the alleged plotters were mainly Americans with no apparent ties to al-Qaida or other foreign terrorist organizations.
Miami U.S. Attorney R. Alexander Acosta said in a statement that the investigation was an ongoing operation and that more details would be released Friday.
Managers of the Sears Tower, the nation's tallest building, said in a statement that they speak regularly with the FBI and local law enforcement about terror threats and that Thursday "was no exception."
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 apparently militaristic group.
The residents said FBI agents spent several hours in the neighborhood showing photos of the suspects and seeking information. They said the men, who appeared to be in their teens or 20s, had lived in the area about a year, according to the AP.
The men slept in the warehouse, said Tashawn Rose, 29. "They would come out late at night and exercise. It seemed like a military boot camp that they were working on there. They would come out and stand guard."
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