100 suspects had been taken in for questioning following the attack at a church in Mombasa on Sunday, police have said.
Local police chief Robert Mureithi however said the "main culprits are still at large". (READ: Terror in Mombasa church)
Two people shot when attackers sprayed gunfire died while undergoing treatment, taking the death toll to six, government officials said.
"Two more victims succumbed last (Sunday) night, we still have 15 others admitted in hospital," local ministry of Health chief Khadija Shikely said. Some of those wounded were young children.
The attack in Likoni, near the port city of Mombasa, came amid heightened warnings of a threat of Islamist violence in Kenya and despite boosted security in major cities.
WESTGATE MALL ATTACK
No group has claimed responsibility, but Kenya has been hit by a series of attacks since sending troops into southern Somalia in October 2011 to battle Al-Shabaab insurgents.
Kenyan troops, which have since joined the African Union force in Somalia, are taking part in a fresh offensive launched this month against Shabaab bases.
The attack on the church comes six months after Shabaab commandos carried out the September massacre in Nairobi's Westgate mall where at least 67 people were killed.
Homegrown groups including the Islamist Al-Hijra group, a radical organisation formerly known as the Muslim Youth Center, operate on Kenya's coast and have been linked to the Shabaab.
Several senior Muslim leaders have been gunned down on the coast, with their supporters accusing the police of state-sponsored assassinations -- claims the security forces deny.