By Jonathan Head
BBC News, Bangkok
Suspicion for the Bali bomb blasts is certain to fall on the regional Islamic movement, Jemaah Islamiah, which was held responsible for the earlier blasts on the island nearly three years ago.
The huge blasts in 2002 tore the heart out of Bali's tourist industry, but they also did something else - they woke the Indonesian government up to the gravity of the threat posed by Islamic militants.
Indonesia's president has vowed to hunt down the attackers
Until then, the authorities had been reluctant to move against an extremist fringe for fear of provoking public anger among the world's largest Muslim population.
But the Indonesian security forces already had good information about the al-Qaeda linked network known as Jemaah Islamiah (JI).
After the 2002 bombs, with the help of neighbouring countries, they were quickly able to capture most of those who carried out the bombings.
They have also brought to justice those behind two subsequent attacks against a Jakarta hotel in 2003 and the Australian embassy last year.
Security at major hotels in Bali and Jakarta has now visibly improved.
JI was dealt another blow when Hambali, the man believed to be its commander, was caught in Thailand and handed over to the US.
However the Indonesian judicial system has been criticised for handing down a relatively light sentence against Abu Bakar Ba'asyir, the movement's spiritual leader.
And in recent weeks the Indonesian government has warned that JI militants were preparing to attack again.
Now it seems that warning has proved tragically accurate, just as Bali's tourist industry was getting back on its feet again.