A letter that tested positive for cyanide was sent to the White House this week, it was confirmed last night.
Initial biological testing came back negative, the Secret Service said, but another test conducted returned a "presumptive positive" for cyanide.
"On Monday 3/16/15, an envelope was received at the White House Mail Screening Facility. Initial Biological testing was negative; however, on 03/17/15, the chemical testing returned a presumptive positive for cyanide," Secret Service spokesman Brian Leary said in a statement on CNN.
"The sample was transported to another facility to confirm the results."
A third round of testing will help to determine whether the envelope addressed to the White House does indeed contain cyanide.
The website Intercept, which first reported the 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.
There were no injuries or exposure concerns for the person at the mail-sorting facility who opened the letter, an unidentified law enforcement official told CNN.
A facility located away from the White House and its surrounding buildings had received the letter on Monday for routine screening.
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 will have no additional comment on the matter.
Meanwhile, it also emerged yesterday that Caroline Kennedy, the daughter of former US President John F Kennedy and the US ambassador in Japan has had death threats made against her.
The US embassy in Tokyo received the threats by telephone last month, with several phone calls made by an English-speaking man, the 'Yomiuri Shimbun' newspaper reported.
The threats also targeted the US consul general in Okinawa, Alfred Magleby, according to Yomiuri and other Japanese and US media reports. Okinawa island is known in Japan as host to the bulk of US service personnel stationed in the country.
American and Japanese authorities are working to ensure the safety of Ms Kennedy and the diplomat, the US State Department said yesterday.
Jen Psaki, State Department spokesman, did not confirm any specific threats, but she said in a statement: "We take any threats to US diplomats seriously." (© Independent News Service)