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The threats came in the mail and to date, there have been 25 letters that warn of nuclear bombs destroying America.

People who got them called the FBI and CBS 2′s Kristyn Hartman learned, the Bureau's Chicago office is leading the investigation.

FBI Special Agent Andre Zavala said, "Yes, they alarmed a lot of people."

Attorney Tracy Rizzo was alarmed. A number of days ago, an envelope, with a Chicago postmark and a hand-written address to her private investigations firm, came in the mail.

The letter inside said, "The Al-Qaeda organization has planted 160 nuclear bombs throughout the U.S. in schools, stadiums, churches, stores, financial institutions and government buildings." It also said, "This is a suicide mission for us."

The writer, who claims to be Osama Bin Laden, tells the reader the nukes are remotely controlled. "It was clear the writer wanted to scare me," said Rizzo, "Yes, it frightened me." Rizzo was one of eight people in the Chicago area to contact the FBI.

Agencies in Oregon, California, Texas and Florida also got the letter with the Chicago postmark.

"There are items in the letter that lead us to believe it is a hoax," said Zavala. "There are several demands documented that aren't fully coherent."

He said much of what's in the message doesn't add up, but in a post 9-11 world, the FBI can't let it go - even if it does appear to be a hoax.

"The FBI investigates every element. We can't take that chance," said Zavala.

So, why the threats?

The writer alluded to how families in Afghanistan have to live. The FBI said it will find that writer, and when it does, he could face 5 to 10 years in prison for each letter mailed. Anyone who's received a similar letter should call the FBI.