Maiduguri - A suicide bomber drove a car packed with explosives into the entrance of the police headquarters of northeast Nigeria's Borno state on Friday, killing four people, including a policeman, and wounding seven, the police commissioner said.
There was no immediate claim of responsibility, but the Islamist sect Boko Haram has been blamed for hundreds of bomb and gun attacks on security forces and civilians over the past two years, mostly in Borno state.
Borno police commissioner Bala Hassan told Reuters by phone from the city of Maiduguri that one policeman and three civilians were killed, and seven police officers were wounded by the car bomber.
"He was driving a Toyota Camry, which he tried to drive right into the station. When he couldn't do this, he detonated the bomb," Hassan said. He declined to speculate on who was behind the attack.
Witnesses said they suspected the death toll could be higher.
"Many people, mostly members of the police, which includes men and women, were killed in the explosion," witness Ali Alhaji said.
Earlier, a police officer at the scene, splattered with blood, told Reuters five police vans had been loaded with the dead. He could not be named because he was not authorised to speak.
Official police casualty tolls from insurgent attacks on them are frequently much lower than witness estimates.
Police stations are a favourite target for the insurgency, which flared up partly in response to police brutality against its members, including its founder Mohammed Yusuf, killed in police custody during a crackdown in 2009.
A coordinated Boko Haram strike on multiple police stations in north Nigeria's main city of Kano in January killed 186 people, most of them civilians.
From being a reclusive clerical movement opposed to Western education last decade, Boko Haram has radicalised and mushroomed to become the main security threat facing Africa's top energy producer, and has linked up with other Islamist groups in the region such as al Qaeda's north African wing.
It is based far from oil producing facilities in the south, although its fighters have successfully targeted the capital Abuja, in the middle of the country, a handful of times.
Nigerian forces shot dead 16 suspected militants in a fire fight with Islamist sect Boko Haram on Tuesday, the military said, the sort of move which sometimes provokes a retaliation from the Islamists.
Earlier on Friday, a roadside bomb killed one person in downtown Maiduguri, Hassan said.