'Cyanide' Letter to WH Actually Contained Smoker's Spit

The suspicious letter sent to the White House on Monday was as dangerous as a warm bucket of spit. Literally.

When the piece of mail -- containing a plastic bag and a suspicious liquid -- was initially tested at an off-site screening facility, the results came back negative for toxins. But another test the next day suggested the envelope might contain cyanide, so it was sent for further testing.

The results of the final testing are in: That liquid is saliva. And it did contain trace amounts of cyanide.

How do investigators think the cyanide got there?

The sender is believed to be a homeless man from the Chicago area who apparently smokes excessively, producing trace amounts of cyanide in his saliva, according to sources familiar with the case.

"Smoking cigarettes is probably one of the major sources of cyanide exposure for people who do not work in cyanide-related industries," according to the Centers for Disease Control.

It is unclear whether authorities will charge the man, who sources describe as possibly have a mental illness.