Biker murder police release CCTV
 |  The footage was taken just minutes before Gerry Tobin was shot  |
CCTV footage Mr Tobin, 35, of south-east London, is seen at a petrol station filling up his motorbike before heading onto the A46 and then the motorway.
It was the biker's last known stop before his death and detectives have appealed for witnesses to contact them.
Warwickshire Police have now received more than 500 calls about the murder.
Mr Tobin had been riding home from the Bulldog Bash biker festival, held at the Long Marston Airfield, near Stratford-upon-Avon.
'Fairly relaxed'
Two shots are believed to have been fired from a green Rover 620, with one of the shots killing Mr Tobin and the other hitting his bike.
Det Supt Lawrence said he was still unsure as to why he was killed but was working on the basis that he had deliberately been targeted.
The CCTV footage shows Mr Tobin with his friends looking relaxed and, at one point, smiling up at the camera.
Det Supt Lawrence said: "You can see... that he looks fairly relaxed. He doesn't look as if he's wary, he's not looking furtively around to see if anybody's with him or watching him.
"That suggests to me that he's not a man who feels under any imminent serious threat.
"It kind of suggests that he had had no forewarning of it and if there had been any event in his past or recent past it certainly doesn't suggest from that that he's on his guard as a consequence - or that he fears that he's about to be attacked in any way.
"In summary I would suggest that he didn't know it was coming."
Hells Angels appeal
Det Supt Lawrence said he had not been able to engage in "meaningful dialogue" with Hells Angels members.
He added: "I think the Hells Angels motorcycle club I'm sure will have theories, if not knowledge, and its around that area that I'm asking them to please share those with us.
"I don't know whether the Hells Angels group have that information. I would certainly like to enter into some sort of meaningful dialogue with them and discuss that. As I've said before, they're reluctant to do that."
He urged biker gangs not to consider a revenge attack over Mr Tobin's death, saying: "I would say, don't even consider any retaliation - that would be entirely wrong. The law must deal with this."
Police are still trying to trace a green Rover 620 which they believe was carrying the gunman.
It was seen accelerating away from the murder scene and overtaking several cars. It was captured on CCTV footage but police have not been able to see its registration number.
Mr Tobin's mother has travelled from Canada to help police build up a picture of his character and past life.
