Police attempting to establish the motive behind Derrick Bird's shooting rampage have confirmed he was the subject of an ongoing tax inquiry.
The 52-year-old taxi driver shot 12 people dead in Cumbria before shooting himself on Wednesday.
Detectives said they were focusing on a number of key issues, including suggestions that he was involved in disputes with fellow drivers.
No suicide note or list of targets was found during a search of his home.
Bird's computer, mobile phones, and a large amount of paperwork have now been removed from the property in Rowrah.
A post-mortem examination will be carried out to establish whether he was under the influence of alcohol or drugs at the time of the shootings.
The senior investigating officer, Supt 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."
Police believe Bird deliberately targeted some of his victims and indiscriminately shot at others during his 45-mile rampage.
The first victim was his twin brother, David, whose body was found in the bedroom of his home in Lamplugh.
Police said Derrick Bird did not appear to have forced his way in.
Bird knew two other victims - his solicitor and a fellow taxi driver - but other people he killed were apparently strangers.
All the Cumbria force's available armed officers - a total of 42 - were deployed in an attempt to contain him before Bird was found dead in remote woodland.
A shotgun and a rifle fitted with a telescopic sight were recovered from the scene and a third firearm, a shotgun, was later found at his home.
Cumbria Police have confirmed that all three were lawfully owned.
On Saturday, police revealed that the Citroen Picasso being driven by Bird had suffered collision damage.
Convicted of theft
They appealed for anyone who witnessed or were involved in a collision with the vehicle on or before Wednesday to contact them.
Further details have also been released about Bird's past dealings with police.
In 1990 he was convicted of theft, but did not serve a prison term.
In the late 1990s he reported a theft from his taxi and another incident of passengers running off without paying.
He was the victim of an assault in 2007 and there was criminal damage to his taxi in 2008. A complaint of assault in 2002 was later withdrawn by him.
To commemorate the victims, a number of memorial events are being held on Sunday.
These include an afternoon outdoor service on the Green at Seascale and an evening service in the garden of St Nicholas' Church in Whitehaven.
The Whitehaven service will be attended by the county's lord lieutenant and the high sheriff and a message from the Queen will be read out.
The Bishop of Carlisle, the Rt Rev James Newcome, said: "I think in times of disaster, immediately and instinctively people will turn to the Church.
"We will offer ongoing help and long-term counsel and support."
Copeland Borough Council has organised a number of civic services for Wednesday, one week after the shootings, which will culminate in a minute's silence at noon.