The attack happened outside the town Mandera near Kenya’s border with Somalia and Ethopia, while the bus was travelling to the capital Nairobi.
Surviving passengers said a group of 10 heavily armed men boarded the bus and separated the Somali and non-Somali passengers.
Passenger Ahmed Mahat told the BBC what happened next.
“The non-Somalis were ordered to read some verses of the holy Koran, and those who failed to read were ordered to lie down. One by one they were shot in the head at point blank range.”
He said that some of the Somali passengers begged the gunmen to save the non-Somali passengers.
A local official said that that three of the non-Somalis were spared when they were able to recite verses of the Koran.
Mombassa crack down
The Islamist group said it carried out the attack in retaliations for raids on mosques in the port city of Mombasa. Earlier this week Kenyan police killed one man and arrested nearly 400 others when searching four mosques in the port. The police claim they were being used to recruit new militants and store weapons.
“The Mujahideen successfully carried out an operation near Mandera early this morning, which resulted in the perishing of 28 crusaders, as a 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.