MOGADISHU, Somalia - An Islamic extremist suicide bomber disguised as a police officer killed at least 18 people at a police academy in Somalia 's capital on Thursday, authorities said. The Somalia -based al-Shabab extremist group claimed responsibility.

Another 20 officers were wounded, some of them seriously, Col. Mohamud Aden said.

The bomber, with explosives strapped around his waist and torso, infiltrated Gen. Kahiye Police Academy and targeted officers rehearsing for Somalia 's Police Day celebrations scheduled for Dec. 20, Capt. Mohamed Hussein said.

The bomber walked into the academy undetected and joined a line of officers before he detonated the explosives under his sportswear, Hussein said.

“He detonated his bomb vest having realized that his presence had drawn suspicion among officers in the queue,” he said.

Officer Farah Omar, who was at the scene at the time of the blast, said the bomber targeted a spot where dozens of soldiers had gathered.

“He wanted to inflict maximum damage,” Omar said.

A relative of one victim said he had recently joined the academy. “He wanted to do something for his country, but today we are burying him,” Abdifatah Ahmed said.

Al-Shabab, which is allied to al-Qaida, carries out frequent bombings and attacks against hotels, checkpoints and other high-profile areas of Mogadishu. The group has been blamed for the massive truck bombing in the capital in October that left 512 dead. Only a few attacks since 9-11 have killed more people.

Al-Shabab has become the deadliest Islamic extremist group in Africa and is increasingly targeted by the U.S. military after the Trump administration early this year approved expanded airstrikes and other efforts against the fighters.

The U.S. has carried out at least 32 drone strikes this year. A strike earlier this week against an al-Shabab vehicle carrying explosives prevented an “imminent threat to the people of Mogadishu,” the U.S. Africa Command said.