Killer Derrick Bird held a gun licence for 20 years before an apparent family feud sparked a rampage which left 12 dead and 11 more injured.
As more stories of Bird's three-hour massacre in Cumbria emerged, police tried to piece together why the quiet taxi driver snapped, taking potshots at random strangers but also targeting his brother and family solicitor.
More than 100 detectives are looking at the full sequence of events and trying to establish what made Bird snap and become a killer.
Taxi drivers said he was involved in an argument on the cab rank in Whitehaven on Tuesday night, and it emerged earlier that he was convicted of theft in the 1990s.
One anti-gun violence campaign group said Wednesday's events illustrated the danger of assuming firearms are safe in the hands of seemingly normal people.
The International Action Network on Small Arms said the majority of high-profile mass shootings around the world had been perpetrated by licensed gun-holders using licensed weapons.
Director Rebecca Peters said: "The idea that only 'bad guys' do things like this is false. People who are not career criminals can turn violent. Having weapons designed to kill people is dangerous.
"The gun lobby says legal gun holders are not a problem at all and what we have to worry about is career criminals. But these are not career criminals, they're normal people, so that's an argument for recognising that just checking someone's criminal background is not enough to prevent homicides."