At least seven people were killed after two buses collided head-on in Nigeria’s southern state of Oyo, traffic police said Friday.

Fifteen others sustained varying degrees of injuries from the incident which occurred Thursday along the Ibadan-Iseyin road in the state, Mayowa Odewo, a spokesman for the Federal Road Safty Corps in Oyo, said in a statement.

Citing a preliminary investigation, the traffic police suspected “speed violations and fatigue” as the cause of the incident. It noted that the police had taken possession of the two vehicles for further investigation.

The traffic police lamented the nonuse of seatbelts by any of the passengers, noting this might be responsible for the injuries and loss of lives in the incident.

Deadly road accidents are frequently reported in Nigeria, often caused by overloading, bad road conditions, and reckless driving.