It's a mystery that began 15 years ago. That's when letters began arriving at schools, non-profit groups, government offices and other locations containing threats and a white powder the sender claimed was anthrax. Several received in 2010 helped the FBI make a connection with the earlier deliveries. As our Sarah Blazonis tells us, the Bureau is now asking for the public's help in putting an end to this one-way correspondence.

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SYRACUSE, N.Y. -- AIDS, 9/11 conspiracy theories, and frustration with home shopping TV shows. The topics covered by a series of threatening letters vary widely, but FBI investigators say they have at least one thing in common: they were all likely written by someone in Central New York.

"The postmark is identical, the handwriting is identical, and some of the language and wording that's been used in some of these has been very similar between all of them," said Special Agent Daniel Capone.

The 21 letters were sent to non-profits, government officials, and TV celebrities to name a few. They were scattered throughout the Eastern United States, with eight or nine addressed to the Syracuse area. Return addresses were included for people the FBI found had no connection to the case. They also included white powder the sender said was anthrax.

"Businesses will respond in some manner, which can include evacuating the whole building, shutting down business for the whole day, or, in the case of schools, the entire school gets emptied," said Special Agent Daniel Capone.

None of the letters actually contained a harmful substance, and now the FBI says it's reached a point in the investigation where it's asking the public for help. It's distributing posters, hoping they could be the key to finding the person responsible.

"We believe that someone out there does know this person and may be familiar with the handwriting and the drawings," Capone said.

The letters often include drawings of bloody daggers and single eyes. There were also gaps of time when none were sent, possibly due to illness or incarceration.

Investigators think they're looking for a man between the ages of 35 and 70 who may be seen as odd or eccentric.

"Maybe not educated in the traditional schooling sense, but very well-read and intelligent. The person may have an odd affect. They may laugh at jokes that no one else laughs at," said Capone.

The bureau is hoping these details sound familiar to someone who can help them prevent others from receiving a delivery no one should have to deal with.

If you think you have any information that can help investigators with this case, call 1-800-CALL-FBI (1-800-225-5324) or visit tips.fbi.gov. Officials are offering up to a $10,000 reward for information leading to the conviction of the person responsible.