While the region is prone to attacks by the Al-Qaeda-linked Shabaab group, coastal police chief Larry Kieng said the attack appeared to be motivated by local grievances.
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"The number of people killed are four and we have established it is a conflict between herders and farmers," Kieng said.
He said the attack happened on Thursday night, and that three suspects had been taken into custody.
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Lamu county, which borders Somalia, has faced a string of attacks by the Shabaab, mostly roadside bomb attacks targeting security forces.
In July nine people were shot and hacked to death in the same region, in what was believed to be a Shabaab attack.
However the Standard newspaper later interviewed residents who believed the violence was related to longstanding land disputes.
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