The Cumbria gunman Derrick Bird deliberately targeted some of his victims and indiscriminately shot at others, police believe.
Detectives said his motivations appeared to be a mixture of "grudge and random".
At least three of his victims - his brother, his solicitor and a fellow taxi driver - were known to him, but others were apparently strangers.
The 52-year-old killed 12 people across the west of the county on Wednesday.
Early that morning, Bird was spotted "lurking" outside the Frizington home of his second victim - solicitor Kevin Commons, who was later found shot dead in the driveway.
In other developments throughout the day:
- Prime Minister David Cameron and Home Secretary Theresa May are to visit west Cumbria on Friday
- The surgeon who treated the 11 surviving victims has revealed five were shot in the face
- 100 detectives are now working on the case
- Bird had a previous conviction for theft, police have revealed
A former schoolfriend of Bird said she saw him repeatedly drive his taxi past Mr Commons' home.
Shop worker Iris Carruthurs, 49, was walking her dogs outside the house at Mowbray Farm when she saw Bird in his taxi.
She said: "He passed me and went down to the bottom of the main road, turned, and he came back up.
"Slowly he drove alongside me and I said 'hiya lad, you alright?'
"He didn't speak, he was in a world of his own, and I just kept on walking.
"He was stationed at the gate on the way up to the tip road. I just left him there.
"I thought he was just normal, I didn't think there was anything untoward. I just thought he had been to the farm and dropped somebody off because he was in his taxi."
Asked if she felt she had a lucky escape, she said: "That's what my friends have been saying, but I haven't sat down and thought about that."
Some reports suggest Bird may have been involved in a dispute with a family member over a will, which could have sparked the shootings.
He is believed to have initially killed his twin brother David in Lamplugh before returning to Frizington, where he opened fire on the family lawyer.
But the daughters of David Bird, Rachel, 28, Tracey, 26, and 19-year-old Katie, have insisted in a statement issued through police that there was no family feud.
"We would like to take this opportunity to say there was absolutely no family feud. Our Dad's only downfall was to try and help his brother."
After killing the solicitor, Bird went on to Whitehaven where he shot dead a taxi driver colleague, before embarking on a seemingly indiscriminate killing spree across a 25-mile area of west Cumbria.
He later shot himself and his body was found in a wooded area at Boot in the Lake District.
Two weapons have been seized by police and are being examined by forensic experts. These are a shotgun and a .22 rifle fitted with a telescopic sight.
The Chief Constable of Cumbria, Craig Mackey, has revealed that Bird was convicted of theft in the 1990s, but that the weapons used were legally held by Bird.
He obtained a shotgun licence in 1995 and a firearms licence for the rifle in 2007.
Mr Mackey described the shootings as "unthinkable".
"This is the most horrific incident I have seen in 25 years of policing," he said.
The Cumbria force is now involved in what is likely to be its biggest ever investigation, which includes 30 crime scenes.
About 100 "highly-trained" detectives are now working on the case, said a force spokesman.
In an unrelated incident on Thursday afternoon, officers were called to Nethertown, near Egremont, by a concerned member of the public.
A 65-year-old man was then arrested on suspicion of possession of a firearm with intent to cause harm.
Home Secretary Theresa May - who will accompany Prime Minister David Cameron on a tour of the area on Friday - said Cumbria Police would be provided with additional funding for their investigation if necessary.
She also said local authorities and charities in the area may receive extra support.
Meanwhile, a surgeon who treated some of the 11 victims who survived revealed five had been shot in the face.
Charles Brett, clinical director of West Cumberland Hospital in Whitehaven, said six victims remain in hospital in the county.
Two are in a serious but stable condition, while the others are described as "comfortable".
A seventh victim is being treated at a hospital in Newcastle, but their condition is not currently known.
The shootings also prevented some mourners from paying their respects to teenager Chloe Walker, killed when a coach carrying schoolchildren crashed on the A66.
People had planned to line the streets of her home town of Frizington as the cortege passed, but were warned by police to stay indoors.
According to relatives of Derrick Bird, his mother is "stunned" by what has happened.
Joy Ryan, who lives in Rowrah, said: "I saw her yesterday and she was just stunned.
"She just couldn't make sense of it. She kept saying she wanted to talk to them, she wanted to talk to her sons."
The community of Hungerford in Berkshire, where 16 people were killed by gun fanatic Michael Ryan in 1987, has offered its help to Cumbria in the aftermath of the shootings.
A West Berkshire Council spokesman said: "We have made contact and indicated that if they need any advice or information we might have, we would be happy to provide it. We haven't had any response back from them yet.
"The incident in Hungerford was obviously a while ago but unfortunately we do have some expertise in this area.
"Things like this are so unusual in this country. It certainly brings back memories for people living in this area."