No suicide note or hit-list have been found at killer Derrick Bird's home, police said today.
But officers have seized documents thought to relate to an ongoing investigation by the taxman into 52-year-old Bird's finances.
Mystery still surrounds £60,000 which Bird told a friend he had in his account.
He admitted to fellow cabbie Mark Cooper he feared going to jail.
On Wednesday father-of-two Bird went on a 45-mile rampage first blasting his twin brother David and the family solicitor Kevin Commons, 60, before targeting fellow taxi drivers at the rank he worked from in Whitehaven.
He then drove through the county taking potshots at apparently random people as they went about their everyday lives.
His killing spree only ended when he took his own life in remote woodland.
Senior Investigating Officer, Detective Chief Superintendent Iain Goulding, said: "Detectives are focusing on several key areas and are working to verify suggestions that Bird was involved in personal disputes with fellow taxi drivers, or others.
"We have also been reviewing his finances and investigating issues of taxation.
"We can confirm that Derrick Bird was subject to an ongoing investigation by Her Majesty's Revenue and Customs.
"While we continue to investigate these issues, we may never fully understand what could have driven Bird to commit such atrocities as he is no longer here to answer our questions.
"Detectives have seized a large amount of paperwork from Bird's house, but no "suicide note".
He added that no written lists of grudges held by Bird had been discovered.
"We were simultaneously faced with investigating 12 murders and 11 attempted murders. Within 48 hours we have established who was involved, the major detail of what took place and are building a picture of what may have driven Bird to do this," he added.
"While our investigation continues at a pace, our thoughts are with the families and friends of those who were killed in the most tragic of circumstances."
Mr Hyde confirmed a third firearm, a shotgun, was found at Bird's home.
"A further gun was recovered at his house, lawfully owned. They were all lawfully owned and possessed."
He added: "It may never be clear what was in his mind when he started shooting.
"We have got a lot more information and that's helping us to understand the 'what' of the investigation, and we are using that to narrow down the 'why'."
Details of Bird's previous brushes with the law also emerged today.
:: In 1990 he was convicted of theft from his workplace but was not sent to prison.
:: In the late 1990s he reported a theft from his taxi and another incident of running from his taxi without paying the fare.
:: In 2007 he was the victim of an assault and he reported criminal damage to his taxi in 2008. he also made a complaint of assault in 2002 but later withdrew this.