But Alberto Gonzales, the attorney-general, said on Friday that the plotting of the "home-grown terrorism cell" never went beyond the earliest planning stages.
"There was no immediate threat," Gonzales said, acknowledging the defendants never had any actual contact with al-Qaeda and did not get their hands on weapons or explosives.
John Pistole, deputy FBI director, said that the discussions to attack the 110-storey Sears Tower - the tallest building in the US - were "aspirational rather than operational".
A south Florida grand jury indicted the men on Thursday. The indictment said the men pledged loyalty to Osama bin Laden's al-Qaeda in order to seek support from it for their desire to "wage war" against the US government and build an Islamic army.
They wanted to "kill all the devils we can", it said.
But a person they thought was an al-Qaeda representative was actually an FBI informant, justice officials said.
The operation was high-profile, but officials denied there was any political motivation ahead of mid-term congressional elections in November, and amid a deep slump in the president's popularity and in public support for the Iraq war.
Critics of the government frequently accuse it of exploiting fear of a repeat of the September 11, 2001 attack.
The seven charged, five of them American, were taken into custody on Thursday following a series of raids on various addresses across the country, including a warehouse in Miami, Florida.
They are also charged with conspiring to "maliciously damage and destroy by means of an explosive" the FBI building in North Miami Beach.
The indictment identifies a man called Narseal Batiste as recruiting and training people for the missions, as well as allegedly meeting a purported al-Qaeda operative to ask for cash, guns, radios and vehicles to help him build "an Islamic army to wage jihad", the document says.
The Miami Herald newspaper reported on Thursday that the men were more "wannabes" than terrorist plotters, and said no actual explosives had been found.
The men were named as Batiste, Patrick Abraham, Stanley Grant Phanor, Naudimar Herrera, Burson Augustin, Lyglenson Lemorin and Rotschild Augustine.
Some of the men were reportedly members of a religious group called the Seas of David. Its leader told CNN that five of his members were among those arrested and that they had no connection to terrorists.
Local media reported that agents raided a warehouse on Thursday in the Liberty City area of Miami.
Residents of the area where the raids took place said FBI agents spent several hours in the neighbourhood showing photos of the suspects to people and seeking information. They said the men had lived in the area for about a year.
Richard Kolko, an FBI spokesman in Washington, said "there is no imminent threat to Miami or any other area because of these operations". He declined to comment further.