A letter addressed to US President Barack Obama containing a "suspicious substance" has been intercepted.
Initial tests on the letter, identified at a remote facility, showed the presence of the lethal toxin ricin.
The letter, which arrived at the facility on 16 April, is related to another laced with ricin intended for a US senator, the FBI said.
Meanwhile, US Capitol Police are questioning a suspect in connection with the tainted letters, reports say.
The Federal Bureau of Investigation (FBI) said the letters addressed to the president and Republican Senator Roger Wicker were both were postmarked Memphis, Tennessee and dated 8 April.
The contents of the letter intended for President Obama are being sent to an accredited laboratory for further analysis, the FBI said, with results expected in 24 to 48 hours.
While there is a heightened sense of alert in the capital after Monday's deadly bomb attacks at the Boston marathon, the FBI said there was "no indication of a connection".
A spokesman for the Secret Service, which protects the US president and his family, said it was liaising with the US Capitol Police and the FBI to trace the origins of the letters.
Ricin, extracted from castor beans, is 1,000 times more toxic than cyanide.
It can be fatal when inhaled, swallowed or injected, although it is possible to recover from exposure.
- Can be fatal when inhaled, ingested or - most dangerously - injected
- One to three castor beans chewed by a child, or just eight seeds chewed by an adult, can be fatal
- The toxin is part of the waste produced when castor oil is made