The criminal court sentenced three others to 15 years in prison for their involvement in the July 17 attack near the royal court in the Sunni-majority Rifaa district south of Manama, the sources added.
The defendants complained in court that they had been "tortured during interrogation", the source said.
The men had been charged with forming an illegal group that "used terror".
The explosion caused by the bomb made from two gas cylinders took place outside Sheikh Isa bin Salman mosque.
Since September 29, a total of 143 Shiites have been sentenced to prison terms of up to life, after King Hamad in August ordered stiffer penalties for "terror acts".
These include a minimum 10-year jail term for an attempted bombing. If such attacks cause casualties, the sentence can be life imprisonment or the death penalty.
A Shiite-led uprising to demand a constitutional monarchy in the Sunni-ruled state across the Gulf from Shiite Iran was crushed in March 2011.
But there have been several bombing attacks since then, and Bahraini Shiites -- a majority in the country -- continue to demonstrate in villages outside Manama and frequently clash with police.
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