The reasons why a taxi driver went on a shooting spree in northern England, killing 12 people, could remain a mystery forever, British police have warned.
More than 100 detectives are continuing their investigations into 52-year-old Derrick Bird's rampage across Cumbria, where he shot more than 20 people, before taking his own life.
Included among the dead were Bird's twin brother, David; his lawyer and a taxi driver he knew.
However many of the others Bird killed were apparently targeted at random during his three-hour attack on Wednesday.
While speculation about Bird's possible motives have ranged from a feud with his brother about their mother's will to arguments with rival taxi drivers, police remain unsure of exactly what sparked the deadly attacks.
Cumbria Police detective chief superintendent Iain Goulding said officers would investigate various possibilities but warned they might never find out exactly what drove Bird to kill.
Random killings
"My officers and I are absolutely determined to get to the bottom of why this happened," he told reporters on Thursday.
"However, it may not be possible to establish all the answers, because we cannot speak to Derrick Bird.
"A key part of the 'why' in this investigation is trying to establish whether those tragically killed were chosen because of a motive, because of a grudge or were simply random killings.
"Our initial assessment is we have a combination of both. But I'm not speculating further at this time."
However the three daughters of Bird's twin brother David - Rachel, 28, Tracey, 26 and 19-year-old Katie - have insisted there was no feud between the brothers.
"We would like to take this opportunity to say there was absolutely no family feud," they said in a statement.
'No family feud'
"Our dad's only downfall was to try and help his brother. We are utterly devastated about the death of our dad."
Bird's first victim was his brother, who was killed while at home in Lamplugh.
Family lawyer Kevin Commons was the next to be gunned down in nearby Frizington before Bird drove to the seaside town of Whitehaven, where he opened fire at a taxi rank.
He then fled south in his taxi, taking deadly potshots at people in various towns before fatally shooting himself in a woodland area near Boot.
Bird's elderly mother Mary was said to be "stunned" by the killings.
"She just couldn't make sense of it," her niece, Joy Ryan, said. "She kept saying she wanted to talk to them, she wanted to talk to her sons."
Gun control debate
Police have revealed that Bird held licences for several years for the shotgun and .22 calibre rifle he used in the shootings.
However while he had no history of mental illness, he had been convicted in 1990 of stealing from his bosses while working at the Sellafield nuclear plant.
The slaughter of so many people by a licensed firearms owner has sparked a debate about the need to tighten Britain's strict gun laws.
Prime Minister David Cameron and Home Secretary Theresa May, who are due to visit Cumbria on Friday, have warned against "knee-jerk" reactions to changing the laws.
However they flagged a review would take place once police finished their investigations.
Britain tightened its gun laws significantly in the 1980s and 1990s following two separate massacres - in Hungerford and Dunblane - which each claimed the lives of 16 people.
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