The White House has confirmed that a letter addressed to US President Obama has tested positive for the lethal bio-chemical ricin.

In addition to this another letter sent to Senator Roger Wicker tested positive for the toxic chemical, and a third described as ‘suspicious’ was sent to Senator Carl Levin.

The letters were discovered at an off-site mail centre run by the Secret Service devoted to opening mail addressed to US lawmakers. The mail centre has been closed for security reasons as the suspicious letters have been sent to labs for further confirmation.

Edwin Donovan, Deputy Assistant Director of the Secret Service, said: “The facility is remote and not located near the White House itself. This facility routinely identifies letters or parcels that require secondary screening or scientific testing before delivery. The Secret Service is working closely with the US Capitol Police and the FBI in this investigation.”

All the letters are reported to have been sent from Memphis, Tennessee with no return address. Despite this, ITV News has reported that a suspect, yet to be named, has been identified and is being questioned as the Secret Service has begun their investigations.

If inhaled, just a small amount of the substance will cause severe organ failure in humans, resulting in death anywhere from six hours to five days post-exposure.

Although the timing of the plot is coincidental, the FBI has stated that there are ‘no indications that this is connected with the Boston marathon bombings’.

In 2001, five people died after letters addressed to US Congressmen laced with anthrax were discovered just five days after the 9/11 attack. It was later announced that no evidence suggested the two events were connected.

Ricin is a waste product made from the production of castor oil. Consuming just 8 seeds from a castor bean is enough to be fatal.

The deadly poison can be inhaled, ingested or injected but recovery is possible if treatment is administered quickly, depending on the amount of exposure.