CHICAGO (WLS) -- A Chicago man is now linked to a suspicious envelope sent to the White House.
The Secret Service is waiting for cyanide tests to come back on that letter.
The envelope was received at a mail processing center away from the White House for routine screening Monday and initial tests were negative for cyanide. A second test on Tuesday returned a "presumptive positive," the Secret Service said.
ABC News reports federal investigators said the man who they think sent it is homeless and may have mental problems. They said he has sent packages with rambling messages and foreign substances to the White House before.
Test results from a third test should confirm on Friday whether the substance is cyanide.
The U.S. Postal Inspection Service is also investigating the case and referred all questions to the Secret Service.
The Secret Service, which is responsible for the safety and security of President Barack Obama and his immediate family, said its investigation into the letter was continuing and it would have no additional comment on the matter.
Suspicious letters often are sent to some of the country's leading politicians, including the president. Some test positive for hazardous substances while others include threats of death or other physical harm.
In June 2013, a West Virginia man was indicted on charges of threatening to kill Obama and his family in a letter that included profanity and racial slurs. A federal judge later dismissed the charges after forensic handwriting analysis conducted by the Secret Service showed that 20-year-old Ryan Kirker of McMechen, West Virginia, didn't write the letter.
Two months earlier, letters sent to Obama, Sen. Roger Wicker, R-Miss., and Mississippi judge Sadie Holland tested positive for the poison ricin. The letters addressed to the president and to the senator were intercepted before delivery, but one letter reached Holland. She was unharmed.
James Everett Dutschke of Tupelo, Mississippi, pleaded guilty in January 2014 to sending the letters and was sentenced to 25 years in prison.
The website Intercept, which first reported on Monday's letter to the White House, said it bore the return address of a man who has sent multiple packages to the executive mansion since 1995, including one that was covered in urine and feces and another that contained miniature bottles of alcohol.
The Associated Press contributed to this report.
Homeless Chicago man linked to suspicious letter sent to White House, investigators say