By John Dodge

CHICAGO (CBS) — Over the past six years, Chicago has become a top potential target for international terrorists, a top security expert said Friday, making the posting of a veiled threat by Islamic radicals on Twitter a higher cause for concern.

“Many terror organizations refer to Chicago as Obama Town,” said CBS 2 security expert Ross Rice, a former FBI agent in Chicago. Under President Obama, the U.S. took out Osama bin Laden, and used repeated drone strikes to weaken terror networks.

In the meantime, the U.S. government on Friday issued a security bulletin to all U.S. law enforcement in light of the overall threat posed by Islamic State, commonly known as ISIS or ISIL.

The ominous post on Twitter, purportedly from somebody connected with Islamic State, shows a photo of the Old Republic Building, 307 N. Michigan Ave., in Chicago and the White House.

The text reads:

#AmessagefromISIStoUS

We are in your state

We are in your cities

We are in your streets

You are our goals anywhere.

https://twitter.com/Sunna_rev/statuses/498239066635124736

The handwritten note is dated June 20.

The security bulletin issued Friday cautioned that there is no known threat to any U.S. cities

The Twitter message seems to imply that Islamic State operatives are in Chicago and other U.S. cities.

Islamic State is responsible for executing kidnapped journalist James Foley. Defense Secretary Chuck Hagel said ISIS an “imminent threat to every interest we have.”

Making things more complicated is the fact that these radicalized men are U.S. citizens or foreign nationals here legally, making it easy for them to travel and avoid suspicion.

Islamic State released a horrific video this week showing the beheading Foley, who was a graduate of Northwestern University.