Homeless Man Linked to Suspicious Letter in Cyanide Probe

— -- A suspicious letter sent to the White House that may contain cyanide has been linked to a Chicago-area homeless man, law enforcement sources said.

Testing on the letter is ongoing to determine what, if any, danger it may have posed.

The letter was received Monday at the White House Mail Screening Center, and after initial biological testing came up negative, subsequent chemical testing came up with the positive for cyanide.

The sample was transported to another facility to confirm the results, the Secret Service said.

The substance was inside a plastic Ziploc-like bag inside an envelope, sources familiar with the case told ABC News.

Charges are possible, depending on whether the material was indeed hazardous, the sources said.