Emergency services say they will review why it took three hours to begin decontaminating "victims" in a mock chemical attack in the West Midlands.
Emergency crews wore protective clothing
Sunday's alert at Birmingham's National Exhibition Centre saw 400 volunteer "casualties" being dealt with by police, fire and ambulance crews.
It began at 0930 BST but casualties were not treated until after midday.
Ch Insp Surjeet Manku, of West Midlands Police, said they would "look at why it took so long".
The event, named Exercise Horizon, was followed by the world's media including Arabic TV station Al Jazeera.
The alert, involving 2,000 people, was the largest mock terror attack staged in Britain.
It was designed to test the responses of emergency crews from the West Midlands, Herefordshire, Staffordshire, Shropshire, Warwickshire and Worcestershire.
Police sealed off the scene outside the NEC within 15 minutes of the incident.
A fire service spokeswoman said 14 fire engines were on the scene by 0947 BST. None of the casualties was treated immediately.
After waiting two hours, several victims tried to escape the containment zone but were apprehended by police officers in protective camouflage suits.
The officers managed to bring the situation under control and all the casualties were gathered together in a tight group.
"Casualties" were held in containment zones for hours
Many were angry and confused at the response of the emergency services, said BBC Midlands Today Home Affairs Correspondent, Peter Wilson from the scene.
Mr Manku called it "a really difficult situation... because you have got to assess the situation as it evolves and you are not going to rescue people if the rescuers themselves are affected".
"These are hard decisions that need to be made and that is the purpose of testing incidents of this nature.
"We need to look at why it took so long, whether decisions were rationally made and whether there were any communication difficulties."
The casualties were forced to strip off their clothes and replace them with bright orange capes.
They entered the decontamination unit, designed to clear any traces of the "nerve gas" from their bodies.
All of them had been through the deconamination unit by about 1400 BST.
West Midlands Fire Service co-ordinator Paul Causer said one of the consequences of the exercise could be the introduction of a rapid response team to deal with major incidents such as terror attacks.
Volunteers were forced to strip off to be "decontaminated"
He said one was being set up in London and he hoped the West Midlands could be the next region to deploy such a team.
Deputy Chief Constable Chris Sims, of West Midlands Police, said: "We have got to minimise the time it takes to get this full canopy of resources deployed, and I am sure we will be talking about that.
"There is going to be a delay. I don't think one knows how long the delay is until you run an exercise like this.
"There will be an artificial delay by it being an exercise.
"These are serious issues and they are going to take some handling.
"It really does test the whole system and everyone in it."
Between 160 and 180 firefighters were on the site to deal with the victims.
But the Fire Brigades Union (FBU) said its members were taking part under duress as firefighters are still waiting for their pay award to be honoured.
Sunday's drill follows simulated chemical attacks in London, Newcastle-upon-Tyne and Abingdon, Oxfordshire.