Seven people suspected of plotting to blow up Chicago's Sears Tower and other targets have been arrested by the FBI.
The streets of Liberty City were busy with FBI agents and vehicles
They were seized during a raid by federal agents on a warehouse in northern Miami. No weapons were found.
The suspects include five US citizens and two foreigners, including a Haitian. They are said to be Muslim.
Reports say the group, which had no apparent links to international terrorism, had been infiltrated by a US agent posing as a member of al-Qaeda.
They were reportedly trying to buy weapons.
"In the past couple of days, the US government has taken into custody seven people who were conspiring to conduct jihad [holy war] in the United States," one official told the Reuters news agency.
"They thought they were dealing with al-Qaeda," he said.
US Attorney General Alberto Gonzales is due to give more details of the raids at a news conference later on Friday.
Neighbours in Miami's poor Liberty City area spoke cautiously of the men, who apparently slept in a warehouse where they were arrested.
One told the Associated Press that the men ran a "military boot camp" from the building.
110 floors, 442m (1,450ft) high
Construction began in 1970, completed in 1973
Tallest building in the US
Third tallest in world, after Taiwan's Taipei 101 and Kuala Lumpur's Petronas Tower
Commissioned by Sears, Roebuck and Company, the world's largest retailer at the time
Designed by architectural firm Skidmore, Owings & Merrill
"They would come out late at night and exercise. It seemed like a military boot camp they were working on there. They would come out and stand guard," said Tashawn Rose.
"They seemed brainwashed. They said they had given their lives to Allah," she added.
The Miami Herald newspaper's City Editor, Mani Garcia, who has been working on the story, said there did not appear to be a direct connection between the suspects and al-Qaeda.
"They have been described to us by sources as wannabes or sympathisers," he told the BBC World Service.
As well as the 442-metre (1,450-foot) Sears Tower, the tallest building in the US, the group were planning to target a federal building in Miami, and some FBI buildings, officials said.
But the seven were not posing any immediate danger, according to the FBI.
"There is no imminent threat to Miami or any other area," said FBI spokesman Richard Kolko.
There were a large number of law enforcement officials involved in the operation, one local businessman told AP.
"There is a ton of guys in uniforms moving around, blocking the streets," said Cedric Thomas.