Two crane operators are killed and two people are crushed to death inside cars below when a crane on top of an under-construction Google building COLLAPSES in downtown Seattle

A construction crane on top of an under-construction Google building in downtown Seattle has collapsed killing four people on Saturday.

The crane at the South Lake Union project collapsed on Mercer Street near Interstate 5 shortly before 3.30pm and crushed six cars below, according to the Seattle Fire Department.

Three men and one woman had died when firefighters reached the scene. It's unclear what caused the crane to collapse.

Fire Chief Harold Scoggins said two victims who died were crane operators and the other two people had been inside separate cars below.

Three people - including a mother and her baby - were also injured and taken to taken to Harborview Medical Center. Another person inside a damaged car was treated at the scene and not transported to hospital.

A spokeswoman for Harborview Medical Center, Susan Gregg, said the 25-year-old mother her four-month-old daughter suffered non life-threatening injuries. A 27-year-old man is also being evaluated for non-life-threatening injuries.