Cumbria gunman Derrick Bird was embroiled in a major tax investigation into a mystery £60,000 deposited in his bank account, according to a friend.
Fellow taxi driver Mark Cooper said the Inland Revenue was investigating Bird for the undeclared money, and the killer feared he would go to prison.
The prospect of being locked up may have been the spark that led him to go on a killing spree in Cumbria, shooting dead 12 people before taking his own life in remote woodland.
"All he said was that they had caught him with £60,000 in the bank," Mr Cooper said. "He said, 'They have caught me with £60,000 in the bank, the tax people'. He just said, 'I'll go to jail'.
"He just asked me if he could handle jail. He didn't want to go."
Mr Cooper, 45, who has known the taxi driver for 15 years, said he was not sure where Bird had got the cash from, but added: "He wasn't tight but he didn't splash the cash. His mother was giving him money. A couple of grand a month. She was supposed to have money."
Detectives believe Bird snapped and killed because of grudges against some of his victims and simply murdered others at random.
Police are investigating which of the 12 victims were on Bird's hit list and which were in the wrong place at the wrong time. Detective Chief Superintendent Iain Goulding, of Cumbria Police, said: "A key part of the 'why' in this inquiry is to try and establish why those killed were chosen. Because of a motive, because of a grudge or simply random killings. Our initial assessment shows we have a combination of both."
Prime Minister David Cameron and Home Secretary Theresa May are visiting the devastated community as it struggles to come to terms with the massacre.
