The Somali Islamist militant group al Shabaab attacked 19 men and 9 women on a bus Saturday, but spared some Muslim passengers, according to The Associated Press.

The three spared for being Muslim reportedly recited verses of the Koran for the militants, according to local security official said, the AP reported.

Al Shabaab ambushed the Nairobi-bound bus outside the town of Mandera, near Kenya's border with Somalia and Ethiopia, according to Al Jazeera.

"The Mujahideen successfully carried out an operation near Mandera early this morning, which resulted in 28 crusaders perishing, as revenge for the crimes committed by the Kenyan crusaders against our Muslim brethren in Mombasa," Sheikh Ali Mohamud Rage, al Shabaab's spokesman, said in a statement, Al Jazeera reported.

"They were attacked in places of worship and in their homes. They invaded the Muslim land of Somalia ... It's our duty to take revenge," the statement added.

The attack was supposed to be retaliation for raids on mosques in the city of Mombasa, BBC News reported.

The 28 shot dead were lined up outside the bus and were shot in a line, according to BBC. Some Somalis were shot after pleading with the gunmen to spare non-Somali passengers.

Kenya's armed forces launched ground and air raids and destroyed a camp believed to have been used by the attackers after the attack in search of those responsible, BBC reported.

Kenya's interior ministry Tweeted that the camp is believed to belong to the attackers involved in the attack, but did not report if any arrests had been made, according to BBC.