Gunmen in northern Kenya fatally shot four construction workers at a hospital site near a refugee camp and the border with Somalia where a militant group is active, police said Saturday.

A group of eight workers were resting Friday when they were attacked, leaving four shot dead at close range, a police official who declined to be named due to the sensitivity of the issue told The Associated Press. The other four workers escaped unharmed, the official said.

The hospital construction site is near Kenya’s largest refugee camp, Dadaab, and the border with Somalia where the al-Shabab militant group is based. Garissa county has in the past been attacked by al-Shabab militants who cross through the porous border.

Local police say the Friday attack may have been staged by an armed group that had warned the contractor to stay away from the area, which they consider their turf.

Northern Kenya has in recent days seen violence that has left several people dead in different locations.

On Wednesday, police at the Mandera border point recovered an improvised explosive device that was about to detonate. Last week, two herders were killed at a watering point, also in the Mandera area, by gunmen. In April, five people were killed in a donkey cart explosion in Elwak town.

The government says security operations in northern Kenya have been increased.

The recent attacks have forced the government to suspend plans to reopen the 698-kilometer (434-mile) Kenya-Somalia border that was closed in 2011, although illegal crossings are still rampant.