Officers from North Yorkshire Police's Road Policing Group were among those called to the scene of a collision near Sherburn in Elmet last night.
And while they were helping a seriously injured casualty, someone stole the flashing lights used as part of the road closures.
In an appeal for information, the Road Policing Group said: "This selfish act puts the safety of the emergency services staff and others at the scene at risk.
"While they may look pretty at the moment, the battery will go flat soon and cannot be charged without the case in our cars, so stealing them was pointless."
Anyone with information on the whereabouts of the distinctive items pictured is urged to call 101.