Police announced the arrest of four more suspects Saturday in the bomb blasts at a McDonald's restaurant and a car dealership.

Police also began beefing up security at churches in central Indonesia after finding explosive materials outside one on Friday.

Three people were killed in the Dec. 5 blasts in Makassar, the capital of South Sulawesi, about 1,000 miles east of the Indonesian capital of Jakarta.

So far, police have arrested 11 people in the blast and are searching for four more. A 16th suspect is believed to have died in the McDonald's blast.

Police chief Maj. Gen. Firman Gani said that two of the suspects are the brother and father of Agung Hamid, the alleged mastermind of the blasts and who remains at large. He provided no details on the other two suspects who were identified as Hairul and Itang.

No one has claimed responsibility for the attacks but police are focusing on a hardline Islamic group, Laskar Jundullah, whose leader, Agus Dwikarna, has been jailed in the Philippines for possessing C-4 plastic explosives.

Laskar Jundullah has been described as the armed wing of the Committee to Establish Islamic Shariah Law in South Sulawesi. It has been linked to Jemaah Islamiyah, the group believed responsible for the Oct. 12 Bali blasts, and the group alleged to have provided men and weapons to Muslims fighting Christians in Central Sulawesi.

Hamid and several other suspects are allegedly part of the group and police have found incriminating evidence in the homes of Laskar Jundullah members, including sketches of a church in the South Sulawesi town of Toraja which the group had planned to bomb.