The letter writer claims to be Osama Bin Laden and says the bombs have been left in schools, stadiums, churches, stores, financial institutions and government buildings. The FBI says 20 letters were received by individuals -- eight to local addresses, the rest around the country.
Chicago attorney and investigator Tracy Rizzo received one.
"The person who sent this to me took the time to hand write out the name of my detective agency and to address the letter to me. Quite honestly, that's the most frightening part of it. I was targeted for this letter," said Rizzo.
The FBI tells ABC7 they do not consider the threats to be credible. But they are investigating to try to determine who is responsible and to make sure they are prosecuted.