Chicago's Sears Tower is shown on Friday, Dec. 2, 2005. Several people were arrested in connection with the early stages of a plot to attack the Sears Tower and other buildings in the US, a federal law official said. [AP Photo]

Seven people were arrested Thursday in connection with the early stages of a plot to attack Chicago's Sears Tower and other buildings in the US, a federal law enforcement official said.

The official told The Associated Press the alleged plotters were mainly Americans with no apparent ties to al-Qaida or other foreign terrorist organizations. He spoke on condition of anonymity so as not to pre-empt news conferences planned for Friday in Washington and Miami.

Miami US Attorney R. Alexander Acosta said in a statement that the investigation was an ongoing operation and that more details would be released Friday. Local media reported that agents were raiding a warehouse in Miami's Liberty City section.

"There is no imminent threat to Miami or any other area because of these operations," said Richard Kolko, an FBI spokesman in Washington. He declined further comment.

Attorney General Alberto Gonzales was scheduled to hold a news conference Friday to discuss the raid. A simultaneous news conference was to be held in Miami.

Cedric Thomas, an owner of Thomas Produce Market, told The Miami Herald the area around his store was teeming with federal agents.

"There is a ton of guys in uniforms moving around, blocking the streets," Thomas said.

Several terrorism investigations have had south Florida links. Several of the Sept. 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 North American terror cell.