Avon & Somerset Police are appealing for spectators with video or photos of the fatal accident to contact them.
The crash happened as 23-year-old cyclist Junior Heffernan came round a bend during the Severn Bridge Road Race.
Here is Acting Inspector Darren Jarvis...
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Elizabeth Martin was walking her pet chihuahua on a lead when they were set upon by a wolf-like dog.
Organisers MTV and Plymouth City Council are asking everyone to make sure they plan ahead for a fun, safe and hassle-free event.
It will be a bright start to the day with some showers. There will be a breeze.