At least 23 people were killed and 108 others injured in a series of suicide attacks in Maiduguri, the capital of Nigeria’s Borno State, police said late Monday.

In a statement, police said preliminary investigations indicate the attacks were carried out by suspected suicide bombers.

Security sources and residents told Reuters that the first explosion occurred at a post office in the city center, followed immediately by another blast at the nearby Monday Market.

Additional explosions struck the University of Maiduguri Teaching Hospital and the Kaleri area, with all attacks taking place early Monday evening.

Police said the situation has since returned to normal, adding that security forces have intensified patrols across the city to prevent further attacks.

An investigation is ongoing to determine the full circumstances of the incidents.