Armed police had no chance to stop Bird before he died
Bird is said to have feared being jailed over a tax offence
Police in Cumbria have said they had no opportunity to end the killings by taxi driver Derrick Bird sooner.
All available armed officers - a total of 42 - were deployed as soon as the shooting was reported.
The taxi driver shot his brother and solicitor first, but it was not until the shootings at a Whitehaven taxi rank that police were alerted.
He then went on a "45-mile rampage", police said, shooting at random while officers tried to contain him.
Derrick Bird, 52, killed 12 people and wounded 11 others on Wednesday before shooting himself.
In a news conference Chief Constable Craig Mackey said the murders were the most "tragic and complex" incident he had dealt with in 25 years as a police officer.
But he said all his officers were "100% committed" to finding out why Bird took the lives of those he killed.
He revealed that 42 armed officers were deployed to the area as efforts were made to contain and apprehend Bird.
"At no stage did any police officer have the chance to end this any sooner," he said.
Mr Mackey said police and RAF helicopters were drafted into the area to find Bird as he drove through the county "and a massive land and and air search began".
"Crucially, in this period Bird was firing shots out of his vehicle, moving from place to place, and was not in any one area for a significant length of time," he said.
The killer's job as a taxi driver meant he had a "knowledge of local roads".
"Bird began using back roads to drive through rural hamlets before entering the Eskdale Valley.
"He was travelling along single-track roads in an extremely isolated area and discharged his weapon several times."
Bird was last seen alive at about 1230 BST when he abandoned his car near Boot with one of his guns inside.
"By this point, we were incredibly close to him and officers surrounded the local area and blocked off all escape routes.
"There was a countywide containment operation in place with armed police and helicopters," said Mr Mackey.
Bird's body was eventually found in woodland at about 1330 BST.
Mr Mackey added: "Twelve innocent people - mothers, fathers, partners and friends - were brutally murdered as they went about their daily lives and I am 100% committed to getting to the bottom of this investigation and finding out why."
Earlier on Friday David Cameron and Home Secretary Theresa May met some of the police officers who responded to the killings.
Cumbria was assessed this year by the Inspectorate of Constabulary as a "successful" force, with relatively low crime rates and above-average levels of public confidence.
But the shootings have called into question a decision made four years ago to abandon a plan to merge Cumbria with Lancashire police - the idea foundered because of a dispute about funding.
A bigger force, it was said, would be better equipped to deal with critical incidents. Vic Towell, Assistant Inspector of Constabulary, has now returned to the issue.
In a carefully worded statement, Mr Towell told BBC News: "Cumbria is a good performing force. Its capacity and capability have been stretched to its limits. And one has to consider whether a different configuration would have allowed a different response."
Although Craig Mackey, Cumbria's Chief Constable, dismissed force re-structuring as a "red herring" that would have made no difference to the outcome of the shootings, the argument for having 43 separate constabularies in England and Wales will again be closely scrutinised.
He said he was visiting to show the government cared, adding that the community would need help through what would be "a very, very difficult time".
Speaking at Workington police station, the prime minister said the communities in west Cumbria had suffered the "most appalling tragedy".
Following the news conference, Deputy Chief Constable Stuart Hyde said that Bird's financial affairs was one aspect of the investigation, as well as "looking at relationships within the family, relationships within work, a whole menu".
Mr Hyde said: "There's a whole number of lines of investigation we are following, clearly including financial."
These involved a number of agencies, but when asked directly whether this included HM Revenue & Customs, he would not comment.
At least three of the people Bird shot dead - his twin brother, the solicitor Mr Commons and fellow taxi driver Darren Rewcastle - were known to him.
Seven people who were shot but survived remain in hospital.
Doctors have said four of those are in a comfortable condition at either Whitehaven's West Cumberland Hospital or the Cumberland Infirmary in Carlisle.
Another has been transferred from the care of Cumbria Trust to an undisclosed hospital.
Two people are being treated at a hospital in Newcastle. One is said to be comfortable, while the other remains in the ICU but is improving.
Some reports have suggested that Bird might have been involved in a dispute with a family member over a will, which could have sparked the shootings.
But speaking to BBC Radio 5 live, Joy Ryan, a relative of Bird, dismissed suggestions of a family feud, saying: "Maybe a row that went on the night before, not a feud."
In a statement issued by police, David Bird's daughters have also insisted there was no family feud.
The atrocity is Britain's worst mass shooting since the events of Dunblane in when gunman Thomas Hamilton killed 16 schoolchildren and their teacher in 1996.