Police have been treated in hospital after being exposed to an unknown liquid - petrol and possibly heroin - during a drugs raid.
Eight officers felt sick and struggled to breathe while aiding colleagues from the Serious Organised Crime Agency (Soca) in Bordesley Green, Birmingham, according to West Midlands Police.
A spokesman said: "Enquiries are ongoing to establish exactly what the liquid was, although early indications suggest it may have been petrol mixed with an illegal drug."
Three men, aged 21, 27 and 34, were arrested on suspicion of conspiracy to supply controlled drugs during the midnight operation.
Officers forced entry into a house on Mill Burn Way and seized suspected Class A drugs with a street value of about £250,000.
A 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."
A total of 11 people, eight police officers and three detainees, were taken to Heartlands Hospital for treatment.
All were discharged from hospital early on Friday morning and the three detainees remain in police custody.
Parts of the area are still cordoned off and road restrictions in place while a forensic probe continues.
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."