Masked gunmen sprayed bullets and hurled grenades at two churches in a northern town in Kenya on Sunday, killing at least 15 people and injuring several, police officials said. It was the latest in a series of attacks in this East African nation suspected of being carried out by al-Qaeda-linked militants from neighboring Somalia or their sympathizers.

The coordinated assaults unfolded in the town of Garissa, a predominantly Muslim enclave about 120 miles from the border that Kenyan forces have used as a base of operations to fight Somalia’s Islamist al-Shabab militia.

“They just walked in and shot the people inside the church after they killed the policemen guarding the entrance,” said Philip Ndolo, the regional deputy police chief, referring to one of the attacks.

Two days ago, gunmen kidnapped four aid workers from Kenya’s Dadaab refugee camp, about 60 miles from Garissa, and killed their driver before apparently fleeing with the hostages into Somalia.

No group asserted responsibility for Sunday’s attacks, but suspicion immediately fell on al-Shabab. Ndolo said police are investigating whether the militant group or its sympathizers were behind the bloodshed but cautioned that there could be other assailants. At the time of the attacks, police were protecting both churches because of concerns that al-Shabab, which has targeted Christians, could strike there.

Ever since Kenyan military forces launched operations inside Somalia in October in an effort to weaken the control of the Islamists, Kenya has experienced a spate of attacks, harming its tourism industry and threatening its economic growth.

Militants have targeted bus stations and other public areas, including a grenade attack on a nightclub in Mombasa on June 24, a day after the U.S. Embassy warned that an attack in the port city was imminent.

Al-Shabab militants were suspected of orchestrating grenade attacks against two churches in March and April in Kenya. But the twin attacks on Sunday were reminiscent of assaults against churches by the Nigerian Islamist group Boko Haram, which has killed scores of Christian worshipers in predominantly Muslim northern Nigeria.

Ndolo said the Sunday attacks unfolded about 10:15 a.m. in the town of 150,000 people. At the Catholic Church, men threw two grenades; one failed to detonate, but the other exploded at the entrance, injuring three. The majority of the congregants were inside the church, attending the Sunday service.

“If the grenades had entered the church, there would have been a lot more casualties,” Ndolo said.

At the African Inland Church, men wearing balaclava ski masks and clutching guns killed the two police officers at the entrance, then opened fire on congregants inside the building, before hurling grenades, Ndolo said. Nine people were killed at the scene, and six died at the hospital. Twenty-five were hospitalized.

“We won’t rest until we arrest those responsible,” Ndolo said.