Eight police officers have been treated in hospital after being exposed to an unknown substance during a drugs raid.
West Midlands Police said the officers experienced sickness and breathing difficulties while supporting colleagues from the Serious Organised Crime Agency (Soca) in Bordesley Green, Birmingham, early on Friday.
A police spokesman said early indications suggested the substance, found at a house in Mill Burn Way, was a mixture of petrol and an illegal drug, possibly heroin.
Three men, aged 21, 27 and 34, were arrested on suspicion of conspiracy to supply controlled drugs during the operation.
Officers forced entry to the premises at around midnight and seized a quantity of suspected class A drugs with an approximate street value of around £250,000.
The police spokesman said: "While the arrests were being made, officers were exposed to an unknown substance which caused ill-effects for both the officers and the people arrested, namely sickness and breathing difficulties.
"In total 11 people, consisting of eight police officers and three detainees, were taken to Heartlands Hospital. All were discharged early this morning and the three detainees remain in police custody, where they are being dealt with by Soca officers.
"Inquiries are ongoing to establish exactly what the liquid was, although early indications suggest it may have been petrol mixed with an illegal drug."
Parts of the area are still cordoned off and a number of road restrictions are in place while forensic examinations continue.
West Midlands Police incident manager, Chief Inspector Lee Wharmby, said: "Police officers face threats on a daily basis and, on occasions, criminals will use extreme measures to resist arrest. 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."