Nine people are awaiting treatment after a suspected chemical leak which forced the evacuation of a council office for the second time in less than two weeks.
The Essex County Council building in The Crescent, Colchester, was first cleared by authorities on June 29 after reports of people falling unwell and reporting a strange smell.
Essex County Fire and Rescue Service said a similar incident had taken place this morning.
A spokesman for the force said: "The building has now been fully evacuated and all people have been accounted for.
"Nine people are awaiting triage from the ambulance service. They are all receiving oxygen therapy from firefighters."
A specialist Detection Identification and Monitoring (Dim) vehicle from Epping - on its first emergency incident - has been sent to the scene to assist in identifying the chemical involved.
Ambulance service spokesman Gary Sanderson said: "At present our team of ambulance crews are working closely with the fire service to ascertain what has caused the sudden illness of the members of public at this location this morning."
Just two weeks ago emergency services were called to the same building following reports of a strange smell in the area and people feeling unwell.
The fire service had been unable to identify any hazardous material.
The East of England Ambulance Service has not been able to identify any particular cause or reason for these people becoming unwell.
In total 29 people were assessed at the scene and then went home, while three were taken to hospital for further assessment.