A US jury has cleared Ahmed Khalfan Ghailani of all but one of over 270 charges linked to the 1998 African embassy bombings.
He is the first Guantanamo suspect to face a civilian court.
Two-hundred and twenty-four people died in the al Qaeda attacks on the US embassies in Kenya and Tanzania.
The New York jury deliberated for five days but found him guilty of only one relatively minor charge of conspiracy to damage or destroy US property with explosives.
It was a rare defeat for the US Attorney’s office.
However, Ghailani still faces a mandatory minimum of 20 years in prison.
The trial has been seen as a test case. President Obama wants a mixture of civilian and military courts to try the 174 Guantanamo prison terror suspects. Most Republicans are calling for military tribunals.