One also told the Sun newspaper that Bird had said "there's going to be a rampage tomorrow" after getting into an argument with other taxi drivers.
Al Jazeera's Tim Friend, reporting from Cumbria, said some clues pointed towards a feud over his mother's will.
"One of the first victims is reported to have been his twin brother. He then went on to shoot the family lawyer," he said.
"Derrick Bird, according to all reports was a very placid man. He enjoyed going to the pub and had recently become a grandfather.
|Media have speculated over a family feud being the motive behind the rampage [AFP]
"Something clearly triggered a violent reaction in him".
The killings - Britain's deadliest mass shooting since 1996 - has left many shocked in a nation where handguns, semi-automatic and pump-action rifles are banned.
In recent years, there have been fewer than 100 gun murders annually across the country.
The shootings began in Whitehaven, a coastal town in the Lake District, a popular tourist destination, on Wednesday morning, and continued through to at least another 18 locations.
Bird's body was found in woods near Boot, a hamlet popular with hikers and vacationers.
Police said two weapons were recovered from the scene and that Bird had been a licensed gun owner for 20 years.
'Grief and horror'
David Cameron, the prime minister, offered condolences to "all those caught up in these tragic events, especially the families and friends of those killed or injured".
He said politicians had been "shocked and alarmed" by the killings.
Queen Elizabeth II said she shared in "the grief and horror of the whole country".
Rules on gun ownership were tightened after two massacres in the 1980s and 1990s.
In 1987, gun enthusiast Michael Ryan killed 16 people in the English town of Hungerford.
In 1996, Thomas Hamilton killed 16 children and a teacher at a primary school in Dunblane, Scotland.