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 tell ABC News.

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.

Sources familiar with the case say told ABC News the substance was inside a plastic Ziploc-like bag inside an envelope.

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