Iran.

That's according to former CIA agent turned whistleblower Robert Baer. In a British documentary 'The Cult of the Suicide Bomber', Baer carried out an investigation into the phenomenon of suicide bombings during the 20 year Israeli occupation of Lebanon.

23 October 1983: 241 American Marines are killed in the most powerful non-nuclear explosion since the Second World War.

Baer was a CIA agent in the American Embassy in Beirut in the 80s when 2 suicide bombs attacked the embassy.

He visited the head of Lebanon's Military Intelligence. Why, Baer asked, did no group claim responsibility for the barracks bombing? Because the country that was responsible doesn't want it to be known, the General replied. So you are blaming a country? I didn't say that, said the General. Can you tell me which country was responsible, Baer asked. I don't have that right, the head of Lebanon's Military Intelligence said.

The fluent Arabic speaker revealed that just before he left the CIA he found secret documents. They said that weeks before the American Embassy bombing, there was evidence that the Iranian Revolutionary Guard was in contact with Lebanese groups with the capability and willingness to bomb the American Embassy,