Chicago: The authorities here and across the United States have tightened security after fears of counterattacks from al-Qaeda after the Osama bin Laden kill, as hints emerged from the materials recovered from the Abbottabad compound that slain al-Qaeda chief Osama bin Laden was plotting to attack America on the tenth anniversary of 9/11.

"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.

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 threat calls to attack the former Sears Tower, now Willis Tower. 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.

"There was never a credible threat on the Willis Tower," Rice explained.

Pak also harboured bin Laden

Pakistani Intelligence reports have also revealed that Pakistan was harbouring Bin Laden, despite Pakistan being a US ally in the war on terror.

We have no credible information to suggest that a specifically targeted plot is under way," Matt Chandler, a DHS spokesman said in a statement.

"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.

We encourage reporting of suspicious activity to appropriate authorities and encourage our homeland security, military, and law enforcement partners, as well as the general public, to remain vigilant," Chandler added.