The US Secret Service is conducting a third round of testing to determine if the envelope addressed to the White House indeed contain the potentially deadly chemical cyanide.

The first testing came back negative but the Secret Service, which is tasked to ensure the safety and security of US President Barack Obama, had to order a third screening because the second testing returned a "presumptive positive" for the highly toxic chemical.

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The letter was sent to a facility that screens letters for the White House. The facility was located away from the office of President Obama. All letters addressed to the White House are screen in the said facility, sources said.

The Secret Service, however, did not say whether the letter was addressed to President Obama or to someone else at the White House. Officials also declined to say whether something specific had led them to test the letter.

"As this is an ongoing investigation, the Secret Service will have no additional comment," the statement said.

The sender of the said envelope has not been named while the investigation was ongoing. Obama has not issued a statement regarding the incident.

It was not the first time that letters containing poisons were sent to the White House.

In April 2013, letters sent to President Obama, Sen. Roger Wicker and Mississippi judge Sadie Holland tested positive for the poison ricin.

The letters addressed to Obama and Wicker were intercepted before delivery but the one addressed to Holland was received. The judge, however, was not harmed.

The man who sent those letters, James Everett Dutschke, pleaded guilty in January 2014 and was sentenced to 25 years in prison.

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.

TagsBarack Obama, Cyanide Letter