Three militants sentenced to death for the 2002 Bali bombings which killed 202 people were executed by firing squad, local television reported.
Amrozi, 47, his brother Mukhlas, 48, and ringleader Imam Samudra, 38, were killed with shots to the heart on the island prison of Nusakambangan off southern Java, TV One television reported quoting an official source.
There was no official confirmation but the attorney general's office in Jakarta has told journalists to expect an announcement.
The bombers were sentenced five years ago for the 2002 attacks on the resort island of Bali which killed more than 160 foreigners including 88 Australians, as well as 38 Indonesians.
They repeatedly said they wanted to die as "martyrs" for their Islamist cause but the executions were delayed by a series of failed appeals and most recently by the Muslim holy month of Ramadan in September.
Security forces are on high alert across the mainly Muslim archipelago amid fears of reprisal attacks.