Officials identify suspect behind cyanide letter sent to White House

A puddle in the driveway reflects the White House and north lawn on September 25, 2014 in Washington, DC.

Mark Wilson, Getty Images

Investigators have identified a homeless man from the Chicago area as the suspect behind the letter sent to the White House that tested positive for cyanide, a U.S. law enforcement source told CBS News on Thursday.

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 return address on the letter, which was received Monday at a facility that screens mail to the White House, was the address of a merchant who told law enforcement that the homeless man uses the store's address to get 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.

There are no reports of mail handlers at the facility being exposed to cyanide.