At least 36 people were killed at a quarry at Koromei in Mandera County on Tuesday morning by suspected Al-Shabaab militants.
Most of the victims are said to have hailed from Chaka in Kiganjo, Nyeri County. Survivors told police Muslims at the site were spared.
The victims were made to line up and most of them were shot in the head while at least two were beheaded.
Police say they were attacked at around 1am as they were sleeping in tents at the site, about 10 km from Mandera town.
The attack happened just four hours after three men armed with AK-47 rifles attacked a club in Wajir, shot dead one person and injured 13 others.
The masked attackers raided Ngamia Club, about 800 metres from the Wajir Police Station, and shot indiscriminately at the revellers.
Mr Boniface Daae was shot in the back and was later pronounced dead. Police spokesperson Zipporah Mboroki said three other victims — Kevin Ojwang, Tom Gikundi and John Muchiri — were admitted to a hospital in serious condition.
State House released a statement on the Mandera and Wajir attacks Tuesday and said investigations into the two incidents have been launched.
The statement by State House Spokesperson Manoah Esipisu indicated that a full statement will be issued later Tuesday.
On November 22 this year, 28 bus passengers were shot dead in an attack at Arabiya, some 50 kilometres from the border town, for which the Somalia-based radical Islamists later claimed credit.
They said the attack was in response to police raids on mosques in Mombasa the previous week.