A Kenyan police official says gunmen believed to be from Islamic extremist group al-Shabab killed one person and severely wounded three others in the northern county of Mandera, which borders Somalia and has experienced a string of attacks in recent months.

The official, who insisted on anonymity because he wasn't authorized to speak with the media, said Sunday the victims are believed to be non-Muslims.

The attack comes two days after gunmen attacked the convoy of Mandera Gov. Ali Roba, killing four people including two policemen. Roba survived the attack.

Al-Shabab, based in Somalia, has vowed to inflict violent attacks on Kenya because it has contributed troops to the African Union force supporting the Western-backed government in Somalia.