“It was revealed that Dorner has become the first human target for remotely-controlled airborne drones on US soil.

A senior police source said: “The thermal imaging cameras the drones use may be our only hope of finding him. On the ground, it’s like looking for a needle in a haystack.”

Asked directly if drones have already been deployed, Riverside Police Chief Sergio Diaz, who is jointly leading the task force, said: “We are using all the tools at our disposal.”