A biker who was shot dead with a single bullet to the back of the head was the victim of a planned attack.
Mr Tobin's bike was a black Harley Davidson
Gerard Michael Tobin, 35, from Mottingham, south London, was riding on the M40 near Leamington Spa when he was killed on Sunday afternoon.
Mr Tobin had earlier visited the Bulldog Bash motorcycle festival at Long Marston airfield.
The Canadian had lived in the UK for 10 years and worked at a Harley Davidson dealership in south London.
Detective Superintendent Ken Lawrence, of Warwickshire Police, said Mr Tobin and two friends had been travelling down the A46 towards the M40 roundabout after the event.
He said that as the three motorcyclists rode past a lay-by on the road, a green Rover 600 series car pulled out and started following them.
Detectives said that several miles after joining the motorway, the green car - which had two or three people in it - overtook two of the three motorcycles and shot at Mr Tobin.
Two shots are believed to have been fired, although only one hit Mr Tobin on his head just below his helmet.
He fell from his bike, which careered down the road for up to 200 yards before veering off the carriageway.
Other vehicles on the road braked hard and stopped but the Rover carried on and was not seen again, Det Supt Lawrence said.
Detectives believe the murderer was travelling in a green Rover car
He added that Mr Tobin, who was in a stable relationship and worked as a mechanic, was "hard-working" and "a man of good character".
Det Supt Lawrence said Mr Tobin had not come to the attention of the police before and there was nothing in his past to indicate why anyone would want to murder him.
He said: "Astonishing is the best word to describe this case.
"It is an incredible story. This man was travelling at some speed - we think about 70mph. I don't know of anything quite like this happening before."
Police had earlier said biker gangs are a "strong line of inquiry" in the investigation, although Det Supt Lawrence refused to go into detail on the way their inquiries were progressing.
The officer did say it "seems likely" this was a planned attack and not a random event.
Looking for motive
"The reason why it should be planned and took place I don't know. That is clearly a key line of investigation that we are pursuing.
"It certainly appears to have been reasonably well-planned in that if that car in the lay-by was in fact there waiting for him to pass... if that was waiting for him, expecting him to take that route, then of course, that does indicate some prior knowledge.
"There were other motorcyclists travelling on that motorway at that time, both leaving the Bulldog Bash and others, and it seems that Gerard Tobin was quite clearly the intended victim."
A notice on the window of the Harley dealership in south London said it would be shut until Thursday and apologised for the inconvenience to customers before thanking them for their understanding.
Warwickshire Police said they had already received more than 200 calls from the public with information about the incident.
However, they urged any more witnesses to come forward who saw the car or Mr Tobin's distinctive black Harley Davidson FXSTB Night Train motorcycle.
A spokesman said the other two bikers who had been in convoy with Mr Tobin were helping police with their inquiries.
Specially-trained officers carried out a fingertip search on the stretch of the motorway on Sunday and Monday and police are now studying footage of the motorway.
Detectives said they were also examining a similar shooting involving bikers from the Bulldog Bash six years ago.
In that incident, three bikers were shot at on the M40 as they returned from the event, but survived.
The person responsible for that attack has never been caught.
The Bulldog Bash is one of the most popular biker parties in Europe and attracts tens of thousands of motorcyclists and music fans from across Europe.