The authorities in Chicago and across the US have stepped up security after clues emerged from the materials recovered from the Abbottabad compound that slain al-Qaeda chief Osama bin Laden desired to attack America on the tenth anniversary of 9/11.
The authorities also tightened security as it feared of counterattacks from al-Qaeda after the slaying of bin Laden.
"There was no plan to attack anything or anyone" but "This was his (Osama's) thought for the future. Maybe his desires," Ross Rice, spokesman for the Chicago FBI said.
In October last year, two explosive packages addressed to Chicago-area synagogues and packed aboard cargo jets were seized. The plot triggered worldwide fears that al-Qaeda was launching a major new terror campaign.
Rice said that Chicago is on the radar line of al-Qaeda because of its central location, big size, infrastructure and tall buildings.
Earlier in 2001, during 9/11 there were rumoured threats to attack the former Sears Tower, now Willis Tower.
"There was never a credible threat on the Willis Tower," Rice explained.
Pakistani Intelligence reports have also revealed that Pakistan was harbouring Bin Laden, despite Pakistan being a US ally in the war on terror.
"In the wake of Osama Bin Laden's death, Department of Homeland Security (DHS) has regularly shared information with their federal, state, local, tribal and private sector partners regarding the potential for unaffiliated individuals to conduct retaliatory attacks in the homeland," Matt Chandler, a DHS spokesman said in a statement.
"Although there are no credible threats or reporting to indicate the active planning of such an attack, bin Laden's death could inspire violent extremist followers or lone offenders to attempt to conduct retaliatory attacks.
We have no credible information to suggest that a specifically targeted plot is under way," Chandler added.
"Our security posture, which always includes a number of measures both seen and unseen, will continue to respond appropriately to protect the American people from an evolving threat picture both in the coming days and beyond," Chandler said.