A letter sent to the White House possibly containing cyanide was linked to a Chicago homeless man, according to law enforcement officials.
Testing of the letter is still ongoing. Initial tests showed the letter was negative for cyanide but a follow up test indicated a positive.
The man is known to FBI, Secret Service, and U.S. postal inspectors. Officials believe the individual might suffer from mental illness because of the rambling nature of the letter, and it appears he may have sent previous letters to President Obama, which put him on law enforcement's radar.
The letter had a return address of a merchant who told law enforcement that the homeless man uses the store's address to receive mail.
CBS News chief White House correspondent Major Garrett reports the letter had what was described as a milky substance inside a container. The initial biological testing came back negative, a Secret Service spokesman said, but further testing revealed a "presumptive positive" for cyanide -- a deadly toxin that prevents cells in the body from using oxygen. The sample has been taken to another facility for further testing.
Handlers at the mail facility were not exposed to any cyanide.