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.