Kenyan churches are tightening security after a lone attacker exploded a grenade inside an evangelical church in Nairobi on Sunday (April 29), killing one person and injuring 15.

Meanwhile, a string of bombings in Nigeria during Sunday morning worship services killed at least four people in Maiduguri and 15 in Kano, with many others injured.

The Kenya attacks at the God's House of Miracles International Church occurred days after the U.S. embassy warned of an impending attack by al-Shabab, a militant Islamist group in neighboring Somalia.