NAIROBI (Reuters) - A Kenyan court sentenced two men on Friday to 18 years in prison for helping gunmen from Islamist group al Shabaab storm Nairobi’s Westgate shopping mall in a 2013 attack that killed 67 people.Mohamed Ahmed Abdi and Hussein Hassan Mustafa are the only people convicted in relation to the four-day assault which showed the Somali militants could strike in one of the most secure areas of the Kenyan capital. They were sentenced for 18 years apiece on each of two charges brought under a national terrorism prevention act, to be served concurrently. Abdi was given an additional 15-year jail sentence for possession of materials promoting terrorism. “This court has to pass a sentence ... commensurate with the offence,” Chief Magistrate Francis Andayi told the men as they stood in a Nairobi courtroom. He said the seven years they had spent on remand would be deducted from their sentences. Andayi said they are allowed to appeal the sentences. The attack on the upscale mall, a favourite of Kenya’s growing middle class and foreign workers, happened two years after the East African nation sent troops into Somalia following a series of kidnappings and raids on Kenyan soil. Four gunmen died during the attack and it has never been proved there were others who escaped. A third defendant, Liban Abdullah Omar, was acquitted in the trial, which concluded on Oct. 7. He was abducted by a group of gunmen a day later after being released from custody and his whereabouts is still unknown. The three defendants -- all ethnic Somalis, two of whom are Kenyan citizens -- were accused of assisting the attackers. Kenyan authorities’ disorganised response to the Westgate attack damaged the country’s reputation. Soldiers and police fired at each other during a chaotic four days and footage emerged of soldiers looting the complex as bodies lay sprawled on the bloodstained floors.