NAIROBI, Kenya (AP) — A donkey cart carrying a suspected improvised bomb blew up at a checkpoint on the Kenya-Somalia border Thursday, killing one Kenyan police officer and critically wounding four others, authorities said.

A Kenyan police report seen by The Associated Press said the cart pulled by two donkeys and ridden by one man passed the Somali checkpoint of Bula Hawa and entered Kenyan territory, where it was stopped by officers to check the load.

The rider jumped off and ran back into Somalia moments before the cart exploded, causing a huge fire at the border post in the northern county of Mandera, the report said.

The report said that the cart’s driver was arrested by Somali police as he tried to flee, and that the Mandera county security team was negotiating with the Bula Hawa police to have him handed over to Kenyan authorities.

No one claimed responsibility for the attack, but suspicion immediately fell on al-Shabab, a Somalia-based extremist group linked to al-Qaida.

Al-Shabab has vowed retribution on Kenya for sending troops into Somalia in 2011 to fight the militants. The group had staged a string of kidnappings of Westerners inside Kenya that threatened the country’s tourism; a key pillar of its economy.

The Kenyan forces became part of the African Union peacekeeping mission that has bolstered Somalia’s weak government for more than 20 years against an al-Shabab insurgency. The AU mission last year began a drawdown of it troops under a U.N. Security Council resolution to return control to the Somali government.

In recent years, al-Shabab attacks in Kenya have been limited to roadside bombs mainly targeting the military and police. On Monday, five police officers were wounded when their truck was hit by a roadside bomb in Lafey Mandera county.