MOGADISHU, Somalia (AP) — Extremists in Somalia attacked a heavily fortified police center in the capital Sunday, killing at least five people before security forces ended the assault, police said.

Four attackers, including two suicide bombers, were also killed in the attack suspected to be by the Islamic extremist group al-Shabab, said police Capt. Mohamed Hussein.

The attack on the police Criminal Investigations Department in Mogadishu started with twin suicide car bombings at the gates of the security facility. After that, at least two gunmen on foot fought their way inside, Hussein said.

Security forces foiled the assault and shot dead the gunmen who tried to storm the heavily fortified facility, said Ali Hassan, a police officer at the Central Investigations Department.

Al-Shabab is waging an insurgency against Somalia's weak U.N.-backed government with the goal of establishing an Islamic emirate, ruled by a strict version of Shariah law.

More than 22,000 peacekeepers are deployed in Somalia in the multi-national African Union force. Al-Shabab opposes the presence of foreign troops in Somalia.

Although al-Shabab was ousted from Mogadishu and most Somali towns in 2011, it continues to wage a deadly guerrilla campaign.