NAIROBI, Kenya (AP) — A military helicopter crash in Kenya near the border with Somalia has killed at least eight people, officials said Tuesday.

It was not immediately clear what caused the crash in the county of Lamu in coastal Kenya. Kenyan defense forces operate in the area to help deter al-Qaida-linked extremists from the al-Shahab group, based across the border in Somalia.

The Department of Defense said the Air Force helicopter crashed while on night patrol. It said a board of inquiry has been sent to the scene.

A defense and a police officials, speaking on condition of anonymity because they were not authorized to speak to reporters, said all military personnel and crew on board the helicopter died.

Kenyan troops are also in Somalia under the African Union Transition Mission in Somalia to help in fighting al-Shabab. The Kenyan forces deployed to Somalia in 2011, but there are now plans to withdraw the multinational forces as Somali troops take over responsibility for their country’s security.

Al-Shabab has increased attacks in Kenya in recent months, killing dozens of people in the border region as the extremists feel pressure from a Somali military offensive that was launched last year.