Intelligence officials have warned Barack Obama that a dangerous regional arm of Al Qaeda is trying to produce the deadly poison ricin to use in attacks against the United States.
Al Qaeda's affiliate in Yemen is believed to have been trying to acquire large quantities of castor beans, which contain the toxic rich protein.
It is thought the poison would be packed around small explosives which could disperse the white powdery substance on detonation.
It is so deadly that even a speck of it can kill someone if inhaled or taken into the blood stream.
The poison was used in the 1978 assassination in London of Bulgarian dissident Georgi Markov and, in 2002, police raided a facility said to be plotting its manufacture in the UK capital.
Al Qaeda intend on detonating the explosives in enclosed areas like shopping centres and airports.