At least six Kenyans had been killed in a grenade and assault rifle attack on a bus in northern Kenyan town of Mandera, witnesses and police said on Thursday. Regional police commander Leo Nyongesa confirmed the incident, saying unknown assailants attacked school bus carrying education officials from Lafey district to Mandera town and killed the six senior officials. \"I think the dead were six but I am still gathering details concerning the attack which occurred early today. We are trying to establish if Al-Shabaab militia are behind the attack but so far it still remains unknown gunmen,\" Nyongesa told Xinhua by telephone from Mandera. Nyongesa said it was too early to ascertain the real motive of the attackers which came eleven days after Kenyan troops launched incursion into Somalia in a bid to dismantle the insurgents blamed for a spate of abductions of foreign on the Kenyan territory. Eye witnesses said those killed in the grenade attacks early on Thursday included senior education officials from the Lafey Secondary School who were transporting examinations papers to schools in Mandera town. \"The officials were heading to pick examination papers from Mandera town to distribute to schools,\" the witness said.The attack came after a senior Kenyan government official reportedly said that Al-Shabaab leaders are seeking negotiations. \"They want to talk,\" said the official, who did not want to be named because he is not authorized to talk to the media told CNN on Thursday. Kenya sent troops into Somalia on Oct. 16 to help subdue Al-Shabaab militants, which it blames for a series of cross-border kidnappings of foreigners in her territory. However, Somali government says it opposes the Kenyan intervention and told Nairobi to restrict her assistance to providing logistics and training for the government soldiers. Analysts believe the military operation launched by Kenya after a spate of cross-border hijackings, the seizure of ship from within its territorial waters and the targeting of tourist havens, may actually revive the dormant terrorist cells.