Inspector Helen Thomas, of West Mercia Police, has told the Shropshire Star: "There was an incident involving a dog and a baby, where the baby was bitten.
"It was not a dangerous dog. It was a small dog and a family pet. We will know more following the post mortem.”
More top news
The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.
A man, apparently armed with a small axe and a large stick, was detained close to the event and police cleared the area as a precaution.