Three dead as two small planes collide in mid-air near Denver

A crashed plane, one of two, lies along Niwot Road between Highway 287 and N 95th Street in Longmont, Colorado (Andy Cross/The Denver Post via AP)

Two small planes have collided in mid-air near Denver, Colorado, killing three people, authorities in the US said.

It was not known what might have caused the crash between the four-seater Cessna 172 and the Sonex Xenos, a light, aluminium, homebuilt aircraft that can seat two.