Qatari businessman Mohamed Salman Haider frequently transferred money between 2010 and 2015, defendants confessed to using his money to execute Sitra explosion in 2015, documents revealed.
Funds were received by the dissolved Al-Wefaq Society, which is responsible for the riots in Bahrain in 2011.
Its Secretary-General Ali Salman is accused of spying for Qatar.
The middle man was Hassan Eissa Marzouq, a former MP and a member of Al-Wefaq Society. He was tasked with providing materials for the bomb and hosting wanted individuals.
Marzouq was arrested after returning from Iran in 2015 and convicted of murder and receiving and providing money related to destructive acts.