The organiser of a fireworks display in Northamptonshire, where 11 people were injured, has apologised to those hurt.
The Health and Safety Executive has begun an inquiry
Fireworks firm Kimbolton, which had prepared the Bonfire Night party at Wicksteed Park, near Kettering, also said it was carrying out a review.
Ronel Lehmann, spokesman for the company, said the firm was "very distressed about what has happened" and "apologetic to the families".
He added: "We invest very, very heavily in safety at firework displays."
About 15,000 spectators were watching the event when a firework shot into the crowd.
The injured, including seven children, one of whom was seriously hurt, were standing behind the safety cordon when they were struck on Saturday night.
The Health and Safety Executive has begun an inquiry into the accident.
The injured were taken to Kettering General Hospital for treatment, mainly for burns to the lower body.
Two children - aged eight and 11 - were detained at the hospital and a third child, a boy aged about 10, was transferred to a specialist burns unit with serious injuries, it was revealed on Sunday.
His condition was described as serious but stable.
Kimbolton has issued an apology to the injured and their families and said it would carry out a review of its safety procedures.
Mr Lehmann, spokesman for the company, said last night's incident had been caused by a malfunctioning "shell".
The firework exploded but did not eject from the mortar tube in which it was encased.
Instead of shooting hundreds of feet into the air, it blew a hole in the tube.
The explosion had a "cascade effect", blowing unexploded shells out of other tubes nearby.
One of these stray mortars ignited and shot towards the crowd.
Mr Lehmann said Kimbolton had run 65 organised displays in the last two weeks.
One of its most high-profile events was the firework display for the Trafalgar celebrations.
Mr Lehmann added: "We have a huge team of people who are dedicated to putting on displays. This is a very sad day for us.
"We will be reviewing the safety and positioning of fireworks and, more importantly, the findings will be imparted to our competitors."
Peter Tait, from St John Ambulance, said a number of children were involved because they were put at the front of the crowd.
Wicksteed Park managing director Mr Roberts said: "All available staff, including our first aid team and St John's ambulance, immediately attended the injured and ambulances were called.
"Six were in attendance plus one from St John Ambulance Brigade."