Extremists in Somalia have attacked the heavily fortified police center in the capital today Sunday, killing at least five people before security forces ended the assault, police has said.

Islamist group al Shabaab claimed responsibility for the assault on the headquarters of Somalia’s Criminal Investigation Department (CID) in Mogadishu.

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

Another 15 people were injured, some seriously, he added.

It was the second major operation in the city this week by the group which has kept up a guerilla war on the Western-backed government in the face of U.S. drone strikes and African peacekeeping forces.

Heavy gunfire was heard out inside for about half an hour after the first blast, said witnesses.

The bodies of four civilians lay in the street near the compound which was partially destroyed. A kiosk close to the wall caught fire.

Al Shabaab’s military operations spokesman, Abdiasis Abu Musab, said one of its suicide bombers had started the attack by ramming a car bomb into the building’s gate.

In al Shabaab’s first attack this week, 13 people were killed when two car bombs went off at the gate of the African Union’s main AMISOM peacekeeping base on Tuesday.

Security analysts have warned that the group could step up its attacks, taking advantage of the distraction caused by campaigning for a presidential election due in August.

Al Shabaab, seeking to impose its harsh form of Islam on the Horn of Africa nation, has also launched attacks in Kenya and Uganda which have contributed troops to the 22,000-strong AMISOM force.

Somalia plunged into anarchy in the early 1990s following the toppling of military dictator Mohamed Siad Barre.