Last Updated Mar 18, 2015 7:25 AM EDT
WASHINGTON -- An envelope addressed to the White House has tentatively tested positive for cyanide after two rounds of analysis, the Secret Service said Tuesday. Additional testing will be necessary to confirm the finding.
The letter was received Monday at a facility that screens mail for the White House and is located away from the grounds of the executive mansion and its surrounding buildings in the heart of downtown Washington.
Initial biological testing came back negative, said agency spokesman Robert Hoback.
Additional testing conducted Tuesday returned a "presumptive positive" for cyanide, a potentially lethal toxin that attacks the heart and nervous system.
The sample has been taken to another facility for further testing.
The letter had what was described as a milky substance inside a container, reports CBS News chief White House correspondent Major Garrett.
There are no reports of exposure to cyanide for the mail handlers at the facility or the building itself, Garrett says.
The Secret Service, which is responsible for the safety and security of President Obama and his immediate family, said its investigation into the letter was continuing and it will 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 Mr. 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 Mr. 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 Intercept website, 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 White House mail screening facility ramped up after letters containing anthrax were sent to the White House and Capitol Hill after 9/11, Garrett points out. The facility processed 1.2 million letters in 2012 and 2013.