"Obviously Chicago in the post-9/11 world has been considered a prime terrorist attack (target)," FBI Special Agent Ross Rice told AFP.
"We're a major metropolitan area. We're a transportation hub. We're a financial hub and our current president of the United States considers Chicago to be his hometown."
Adding to the threat is the fact that during Obama's term in office al Qaeda leaders Osama bin Laden and Anwar al-Awlaki were killed by US forces and Obama also oversaw the Afghan surge and drone missile attacks in Pakistan and Yemen, Rice said.
"So there is a potential threat and then you add to that the heads of state of 40 to 60 different countries are all going to be here," Rice said during a media tour of a joint information centre set up ahead of the summit.
The FBI has been working with the US military and other intelligence agencies to determine if there are any "attacks or incidents planned in connection to the summit," Rice said.
"Right now there are no credible threats involving the Nato summit. We're going to continue to monitor so that that situation won't change," he added.
The Chicago police will be out in force in anticipation of the thousands of protesters expected to take to the streets.
Police and protest organisers have vowed that there will be no repeat of the trouble that erupted at G20 summits in London and Toronto or the riots that scarred Chicago during the 1968 Democratic National Convention.
However the police department will have sound cannons and officers in riot gear ready to deploy should serious problems occur.
The decision to move the G8 summit -- set for Friday and Saturday -- from Chicago to the presidential retreat of Camp David outside Washington is expected to lessen the intensity of demonstrations in Obama's adopted home town.
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