A man and a woman found dead in a car in an apparent suicide pact left signs on the vehicle warning of toxic chemicals inside.
The pair were found dead in a blue Vauxhall Astra parked near to industrial units in Braintree, Essex.
Crews were called to the "chemical incident" at 8.25am today after a worker raised the alarm.
Eye witnesses said notes were placed on the windows of the vehicle warning passers by not to open the doors.
Susan Meads, an administrator at the Braintree Enterprise Centre, where the vehicle was found, said: "The people in question have posted signs on the windows of the vehicle to say there's toxic chemicals inside and not to open the door.
"I believe they have been made on a computer."
Firefighters said hydrogen sulphide was thought to have been used.
Dozens of police, paramedics and fire crews, many wearing protective clothing, attended the scene.
The immediate area was evacuated but fire crews said there was no wider risk to the public.
Essex Police said the deaths were not being treated as suspicious.
Ms Meads said the bodies were discovered by a man who pulled up alongside the vehicle at about 7.30am.
"He thought it was suspicious so he called a colleague, who is an ex-police officer," she said.
"Then they called the emergency services."
The vehicle, in a car park on the site, remained sealed off as investigations continued, she said.
"I arrived for work just as the police were arriving," she added.
"I just think it's a very tragic thing. It's a very sad world we live in."
Some of the 30 businesses on the site were forced to close for the day as police blocked entry to the area.
Divisional Officer Ray Skinner, of Essex County Fire and Rescue Service, said: "We believe the chemical involved is hydrogen sulphide but that has still to be confirmed.
"There is no risk to the public as the car is in a remote location at the end of an industrial estate.
"The area is now a police crime scene and once medical experts have had a chance to do their work, the fire service will be assisting police with recovery at the scene."
Firefighters said a "detection, identification and monitoring vehicle" was being drafted in from Kent to determine the chemical involved.
A spokeswoman for East of England Ambulance Service said around 12 paramedics were on stand-by to treat anyone affected.