Two off-duty emergency service workers were sprayed in the face with an unknown substance in what police have called a "shocking incident".
A police officer and paramedic were among other Good Samaritans who stopped to help a car that had collided with a lorry on the M11 on Friday at around 7pm and was on fire.
Essex Police said as those inside the stolen blue VW Golf got out of the vehicle, they sprayed an unknown substance into the faces of the two off-duty emergency responders.
Chief Superintendent Luke Collison said: "This was an shocking incident and we have launched a significant investigation to track down the suspects and bring them to justice.
"Good Samaritans who witnessed a serious collision stopped to provide urgent medical care and were met by two dangerous offenders and seriously assaulted."
The two people sprayed received medical treatment but it is not thought they will suffer from any long term effects, and the driver of the lorry was uninjured, police said.
Kevin Brown, of East of England Ambulance Service, said: "This unprovoked attack on two Good Samaritans who were trying to help at the scene of a road traffic collision is deplorable.
"Sadly they were in the wrong place at the wrong time and we wish them a speedy recovery."
Essex Police said the suspects left the scene in a white Vauxhall Astra which was taken from a member of the public at the scene of the collision.
This was later recovered in Dagenham, east London.
Officers said one of the suspects involved in the collision is described as Asian and aged between 25 to 30 years old, with a dark beard.
He was wearing a baseball cap, a navy blue buttoned top, and dark coloured bottoms, and also had with him a small, black leather bag.
Another suspect is described as white, with short dark hair. He was wearing a khaki coloured jumper and jogging bottoms and is thought be aged between 20 and 25 years old.
Shortly before the incident on the M11 near Chigwell and Loughton, officers were alerted to the theft of another blue VW Golf from outside a shop on Queens Road, Buckhurst Hill at around 6.50pm.
The victim, a man in his 20s, told officers that one of the two men who stole the car had a knife in his hand - prompting him to run from the scene.
With no verbal or physical threats issued, Essex Police said the two suspects then drove away in the victim's car.
The force said they believe the suspects involved in the car theft on Queens Road may have been associated with the suspects driving the stolen car that collided with the lorry on the M11.
Essex Police said the blue VW Golf which caught fire was not the victim's car, and another blue VW Golf, containing additional suspects, was present when the other vehicle was stolen on Queens Road.
It then followed the other suspects in the victim's vehicle onto the M11 and that it was this car which caught fire. Police said the victim's vehichle is yet to be recovered.
Detectives at Loughton CID are appealing for information and can be reached via 101 or Crimestoppers anonymously on 0800 555 111.