Six people were injured when a bomb exploded at a bus terminal in the eastern Indonesian city of Ambon, local media and police officials said on Thursday.
The blast took place on late Wednesday, on the anniversary of a pro-independence separatist group in the Maluku province.
Maluku police spokesperson Tommy Napitupulu said six people were slightly injured in the bombing.
He said an investigation was underway to determine whether the outlawed secessionist group, the Republic of South Maluku (RMS), was involved in the blast. Four witnesses had been questioned in connection with the explosion.
The separatist group was crushed shortly after its declaration on April 25, 1950, but re-emerged following the downfall of ex-dictator president Suharto in 1998.
A number of the separatist group's flags were seen in several locations in Ambon, the Maluku's provincial capital, on Wednesday morning. Two people were arrested as they attempted to raise flags of the outlawed group.
In 2005, a parade by pro-independence supporters in Ambon triggered a week of clashes between Muslims and Christians, claiming the lives of at least 40 people and injuring dozens of others.
Ambon, about 2,200 kms northeast of the capital Jakarta, and nearby islands in Maluku province, were wracked by three years of fighting between Muslims and Christians that left more than 6,000 people dead.
But the regions have been largely peaceful since the warring party leaders signed a peace accord in February 2002 in order to end the conflicts.