Abdel Raouf Abu Zaid told privately-owned Sudan Tribune website that “the authorities released him after they received documents from the US showing that he had paid monetary compensation to the family”.

In June 2009, a Sudanese court convicted Abu Zaid and four other men in connection with the diplomat’s killing, and handed four of them the death penalty, including Abu Zaid.

His release came after he threatened to go on hunger strike over “inhumane treatment” by prison authorities.

The US said “our embassy is engaging government officials to obtain more information” following the man’s release.

Mr Granville was an employee of USAid when he was killed alongside a Sudanese colleague in Khartoum in 2008.