Militants attacked a Bahraini police bus near the Jidhafs area outside the capital Manama, killing one policeman and wounding eight others, the interior ministry said on Friday. The attack targeted the bus on the Khalifa bin Salman highway, the ministry said, adding that the militant group used a handmade bomb.

“Investigations are underway to determine the circumstances of this premeditated terrorist attack and arrest the group involved,” the ministry said in a statement on its website. The incident was the latest in a series of attacks targeting policemen in the country where the US Fifth Fleet is based.

The eight wounded policemen sustained serious to medium injuries and were admitted to hospital, the ministry said on its Twitter account. It said the attack was carried out by a “terrorist” group which remotely detonated a home-made bomb.

The attack took place near the Shiite town of Jidhafs outside the capital Manama. Authorities opened an investigation into the attack and police began a manhunt to find and arrest the assailants, the ministry said. Bahrain, a Shiite-majority kingdom ruled by a Sunni dynasty, has seen sporadic violence since a crackdown in 2011 on a protest movement demanding a constitutional monarchy and an elected prime minister.

Authorities have since tightened their grip on dissent, jailing hundreds of protesters and stripping a string of high-profile activists and clerics of citizenship. Last week, Interior Minister Sheikh Rashed Al-Khalifa accused Iran of harboring 160 Bahrainis convicted of terrorism and stripped of their citizenship.

He accused Iran’s elite Revolutionary Guards of having trained the group, who were convicted of attacks that killed 25 security personnel and wounded 3,000 others. Bahrain is home to the US Navy’s Fifth Fleet, and a British army base is currently under construction. The government blames the attacks on militants it says are backed by Iran to destabilize the country.

This month, a blast wounded five policemen on Budaiya road, near Manama, while they were guarding a procession by Shi’ite Muslims marking the annual Ashura festival. Authorities opened an investigation into the attack and police began a manhunt to find and arrest the assailants, the ministry added.

An outlaw Bahraini group claimed on social media that a roadside bomb targeted a bus carrying special forces. But a security source said that the targeted bus had policemen on board and not special forces, who don’t use buses for transportation. On October 2, a terrorist blast wounded five policemen on Budaiya road, near Manama.