Eight police officers were taken to hospital after a drugs raid in which suspected dealers poured heroin mixed with petrol on to a carpet, provoking vomiting fits.
The officers suffered sickness and breathing difficulties after seizing a £250,000 haul of suspected Class A drugs from a property in Birmingham at around midnight last night.
As they were arresting three men on the premises, a petrol can containing what was thought to be heroin and petrol was tipped on to the floor, filling the room with toxic vapours.
The three detainees were also affected by the substance and were taken to Heartlands Hospital along with the officers.
Inquiries to establish what the liquid was are ongoing, but West Midlands Police said it may have been petrol mixed with an illegal drug, possibly heroin.
The force carried out the raid in Mill Burn Way, Bordesley Green, in support of officers from the Serious Organised Crime Agency.
They arrested a 21-year-old, 27-year-old and 34-year-old on suspicion of conspiracy to possess a controlled drug with intent to supply.
All the officers and suspects were discharged from hospital early today and the suspects remain in police custody.
Parts of the area around the property remained cordoned off and a number of road restrictions were in place.
But firefighters who assessed the scene after the incident found the substance posed no further risk.
Police said it was unclear whether the mixture had been designed to be used as a weapon or if the heroin was being mixed with the petrol as part of the drug-making process.
Chief Inspector Lee Wharmby, the force's incident manager, said: "Police officers face threats on a daily basis and, on occasions, criminals will use extreme measures to resist arrest.
"What happened overnight demonstrates the ongoing commitment and bravery of officers as they work to keep our streets safe.
"Events such as this always raise concern in communities, but I want to be clear that this is about drugs alone and not connected to terrorism."
The incident comes after police in the city carried out a series of coordinated raids in March in their efforts to target two organised crime groups behind the supply of heroin and cocaine in the area.