Zarkasih is believed to have joined the mujahideen in Afghanistan
Zarkasih - also known as Nuaim, Mbah, Abu Irsyad and other aliases - is believed to be the leader of the South East Asian militant group Jemaah Islamiah (JI).
He was captured in Yogyakarta in July 2007, just hours after the arrest of Abu Dujana, another key JI figure.
In April 2008 both men were found guilty of conspiracy
to commit terrorist attacks, harboring fugitives and
stockpiling illegal arms.
The arrests were said by analysts to be a major blow for the organisation.
News of Zarkasih's arrest was initially kept quiet while the police attempted to round up other JI suspects.
Col Petrus Golose of the national police force described Zarkasih at that time as "the emir of Jemaah Islamiah".
"He controls everyone; everyone important reports to him," he said.
In an apparent video confession screened shortly after his arrest, Zarkasih said he was appointed as caretaker leader of JI after a "leadership vacuum" emerged in 2004.
Very little is known about Zarkasih himself. He is believed to have joined the mujahideen in Afghanistan to fight against the Soviets before staying on to help establish the Taleban government.
Prosecutors also allege that he received military training in Pakistan and has connections with al-Qaeda.
Zarkasih has been accused of arranging the movement of arms and explosives to other militants - Abu Dujana said in court that he always consulted Zarkasih before arranging any delivery of weapons.
However, although Zarkasih is a more senior leader in JI, analysts have said that Abu Dujana probably has more influence on rank and file JI members.