Assistant Chief Constable Steve Cotterill said "please do not see these two arrests as the police inquiry coming to an end" and appealed to the public for additional information.
He said: "I need additional information from the public out there to really get to the truth to what was the reason behind that fire - whilst I know the cause I do not know the reason behind it and I'm still anxious for that information to come forward."
More top news
Abigail Wyatt is charged with causing a public nuisance in connection with the closure of a bridge on the M5 that caused massive tailbacks.
The actress says Depp's decision to pay the charities directly meant he must donate £10 million to her chosen charities.
The judge who handed Oscar Pistorius a six-year murder sentence said the attempt by state prosecutors had no reasonable chance of success.