Detectives who arrested people following swoops on homes early yesterday have released the suspects as inquiries continue.
Seven people were arrested on suspicion of conspiracy to supply heroin.
A 35-year-old man and a 27-year-old woman were arrested in Hopper Terrace, Shotton Colliery.
A 40-year-old man was arrested in Rugby Street, Hartlepool; a 37-year-old man and a 33-year-old woman were arrested in South Parade, Hartlepool.
A 28-year-old man and a 59-year-old woman, were arrested in Sandy Lane, Liverpool.
They have since been released on bail as inquiries continue.
A police spokesman said in the aftermath of the raids: "The suspects were arrested this morning by officers from Durham Constabulary’s specialist crime operations unit, assisted by police officers from Cleveland and Merseyside.
"They are being held for questioning at police stations in Durham and Liverpool.
"The arrests follow an inquiry which began last summer after a drug was seized when a vehicle was stopped in the Shotton Colliery area."
Anyone with information which can help the investigation can call police on 101 or Crimestoppers anonymously on 0800 555 111.