<p>NAIROBI, Kenya — A Kenyan official says four passengers were killed and dozens wounded when a bus hit an improvised bomb on a main road in the northern county of Mandera, near the border with Somalia.</p>
<p>The bus was heading to Mandera town when it ran over an improvised explosive device, Mandera governor Ali Roba confirmed Wednesday.</p>
<p>The force of the explosion ripped the roof off the bus, according to pictures taken at the scene posted on social media.</p>
<p>No group has claimed responsibility for the attack, but suspicion has fallen on al-Shabab, Somalia’s jihadist rebels who are allied with al-Qaida and who carry out many such attacks in Kenya.</p>
<p>Al-Shabab has vowed retribution on Kenya for its troop presence in Somalia since 2011. Kenyan soldiers are part of the nearly 20,000 African Union forces in Somalia to bolster the country’s weak government against the al-Shabab insurgency.</p>
<p>Al-Shabab has carried out numerous attacks on Kenyan soil since 2011 but recently the rebel group’s activities have been centered around five Kenyan counties that border Somalia — Wajir, Mandera, Garissa, Tana River and Lamu.</p>
<p>Mandera governor Ali Roba has said his county was under growing threat from the extremist group which he said controls 50% of the roads and 60% of the county’s territory, forcing the closure of many schools affecting thousands of students.</p>
