A controlled explosion in a Nottingham house was prompted by information from one of two people arrested at the scene, police have said.
Houses and a school around Central Avenue in Mapperley Top were evacuated after a chemical substance was found at a property on Thursday.
A 26-year-old man and a 23-year-old woman arrested are still in custody.
The substance has been sent for forensic analysis but has not been formally identified.
Bomb disposal squad
Officers said they were also examining two computers seized from the flat.
Police entered the property at 07:30 BST on Thursday and executed a search warrant in relation to the theft of a vehicle.
During this, officers found the chemicals and decided to withdraw. A bomb disposal squad from RAF Wittering in Lincolnshire arrived in the early afternoon
Pupils at Mapperley Plains Primary School were taken to the Mapperley Social Club as a precaution and about 60 residents were taken to Richard Herrod Centre in Carlton.
Ian Scott, who lives close to the property which was raided, said he and his family had been given 20 minutes to leave.
"We just didn't know what to take. We asked how long it would be for and we were told it could be two hours, it could be until the evening and maybe even overnight.
'Not taken lightly'
"We have a young son and a dog and that means taking a lot of stuff and we didn't know what was going on but we just thought, 'we have to get gone'."
Supt Mark Holland explained the scale of the police operation.
"The controlled explosion was as a result of an emergency interview with one of the detainees who pointed out a particular substance."
He added: "The decision [to clear the area] is not one we take lightly and we are really grateful to the school and all the residents for their understanding.
"When the facts of the case are known people will fully understand the actions we took."