Three police officers tried to chase Derrick Bird during his shooting rampage in Cumbria but could not stop him, it has emerged.
An unarmed officer based at a Whitehaven police station heard shots at 1033 BST on Wednesday.
He got into a passing car and followed Bird, who was in his own taxi, as he shot another driver. Two other unarmed officers followed in a police van.
Bird killed 12 people on the rampage in west Cumbria before shooting himself.
He also wounded 11 others as he drove for 45 miles through the Cumbrian countryside.
Paul Goodwin had seen the shooting at the taxi rank and decided to follow in his car.
He said: "Just as we got to the corner there were policemen coming down from the police station and I saw the local town bobby so pulled over and shouted 'get in the car - it's him in the taxi'.
"We got to the traffic lights, and we're probably about 10 or 15 yards behind and there's a man walking round the corner.
"I saw him jolt back and put his hands on his face and there was blood there.
"I said, 'it's Paul, it's another taxi driver'."
Deputy Chief Constable Stuart Hyde said that "at no point" did officers "have the opportunity to end the killings sooner".
He added that the first officer went to the assistance of the injured man, who had been in another taxi, and his female passenger, who had also been shot.
A police transit van, with two unarmed officers, then joined the pursuit and continued to follow Bird.
Bird appeared to pull into a driveway, turned and pointed the gun directly at the officers, before driving off at high speed.
The officers attempted to follow but were unable to locate him.
Cumbria Police said in a statement: "They were forced to protect themselves after it became clear they could not reverse due to the traffic that had built up behind them.
"Despite having just witnessed a shooting and having a gun pointed at them, the officers attempted to follow him and despite asking passers-by where he had driven, were unable to locate him again.
"These officers were at the time unarmed and in a vehicle not suitable for a high-speed pursuit."
Having already killed his twin brother, the family solicitor and a colleague, Bird went on to kill nine other people before his body was found in remote woodland.
Mr Hyde said: "Our officers are expected to deal with difficult and challenging situations.
"This incident was unprecedented and exceptional circumstances were fast-moving and highly dangerous.
"Had any officer or member of staff had the clear opportunity to stop Bird, I am confident they would have taken it.
"Those officers were putting themselves in imminent danger, they could see the man was armed and he had just shot someone in front of them."
Inquests into the victims' deaths were opened and adjourned in Whitehaven earlier.
David Roberts, the North and West Cumbria coroner, said initial post-mortem examinations revealed that all Bird's victims died from "wounds from firearm".
An inquest into Derrick Bird's death, in woodland at Penny Hill Farm, in Boot, was also opened and adjourned.
Deputy Chief Constable Hyde commended officers for getting to the scene "within minutes, possibly seconds" of the initial call, adding it was a "pretty fast response in anybody's book".
"These are ordinary neighbourhood officers who did everything they could to get to the scene," he said.
"Armed officers were en route."
He said 42 armed officers from across the county were ordered to go to the scene within minutes of the first call.