CHICAGO -- A federal official said seven people have been arrested in connection with the early stages of a plot to attack Chicago's Sears Tower and other buildings in the U.S. NBC News has learned from federal law-enforcement sources Thursday that the FBI has arrested about half a dozen men in Miami as part of an ongoing terrorism operation. The men, part of a radical black Muslim group, were planning terror acts in Miami and Chicago, the officials said. An FBI informant infiltrated the group, according to the source, neutralizing the threat.