Police increasingly believe that anti-women propaganda may have fuelled anger in the Plymouth gunman before his attack, with his links to the “incel” movement a main strand of their investigation.

Jake Davison, 22, shot dead his mother and four other people on 12 August. It emerged that he had engaged with extremist ideology including the “involuntary celibate” movement, though this is not considered to be enough for security officials to declare the mass shooting as a terrorist attack. Counter-terrorism officers continue to aid the investigation as detectives trawl through the attacker’s electronic devices.

The Guardian has obtained CCTV footage of Davison from the day before the attack, showing him pacing around a convenience store for more than five minutes. The shopkeeper who recorded the clip said it suggested Davison was “clearly was not in his right mind”.

The footage, captured just before noon last Wednesday, shows Davison picking up items, putting them back and scratching his head. “I think he’s confused. He doesn’t know what he wants, and what he wants to do, it is not a big shop,” said the shopkeeper, who asked not to be named.

Davison spent more than five minutes in the shop near his workplace before finally deciding what to buy – a GBP1.35p Monster energy drink and a 39p Biscolata biscuit. He regularly bought food and soft drinks there but never talked to anyone or lingered for long in the store, the shopkeeper said.

A day after the CCTV was recorded, Davison took his legally owned shotgun and killed his mother, Maxine, at the home they shared in Keyham, before killing a three-year-old girl and her father, a dog walker and a woman outside a hair salon. He killed himself as police closed in.

Detectives quickly described it as a domestic incident but as the investigation has progressed, the motivation has appeared more complex. “It is very rare to have a domestic incident when the gunman shoots the family member and then goes on a marauding firearms attack shooting members of the public they do not know,” a source said.

A “determined investigation” into the extent of the role that incel propaganda played will continue, with the misogynistic ideology and mental health at the top of the list of theories behind the motive.

Mental health issues were not known to police when Davison was granted a firearms licence in 2017, nor when it was reinstated last month after his gun had been taken away following a fight.

Sources with knowledge of the thinking of counter-terrorism investigators say there may not be enough evidence to declare the mass shooting as a terrorist atrocity. They point to the fact there were male and female victims, and to a lack of explicit linkage on Davison’s social media or a note advocating or urging violence, or anything suggesting he had been incited by the hate material he consumed.

It may be left to the inquest into the deaths of those he killed to officially determine the issue of motive, and whether this was the first deadly incel attack in the UK.

The shopkeeper with the CCTV footage said: “No one takes that long to buy an energy drink and biscuits. He clearly was not in his right mind. On Friday when I checked the footage, I realised that that’s not normal behaviour.

“On previous days he never took that much time. He would usually get some snacks and a Monster energy drink or a Slush Puppie and some sandwiches. This was very unusual, usually he would be one minute or two minutes max … It’s like something is going on in his head.”

He added: “Usually the regular customers talk with me, but with him there was never anything like that. I felt he was a very introverted person. When I saw that it was him who did the shooting I was so shocked. I couldn’t sleep that night because I served him so many times, and thinking this guy could do this was horrible.”

The shop assistant who served Davison that day also noticed how long he was taking. She said: “It did come across as odd. All I’ve been doing since [the shooting] is just shaking my head, it’s just breathtaking. YouTube videos of him have [been] plastered all over Facebook, and hearing his voice again just kind of makes me shudder.”