Ten people are still being questioned after a newlywed policeman was dragged along by a vehicle before he died.
PC Andrew Harper, 28, from the Roads Policing Proactive Unit based at Abingdon police station, was killed as he responded to a “serious incident” near the A4 Bath Road at the village of Sulhamstead in Berkshire at about 11.30pm on Thursday.
His cause of death has not yet been confirmed, but police say they are working “hard and diligently to find out what happened”.
Two white tents have been erected and forensics investigators have been gathering items and taking photographs.
Officers were seen searching the inside of a grey BMW that was at the scene, according to witnesses.
The 10 people, boys and men aged between 13 and 30, have been arrested on suspicion of murder.
They were all arrested within an hour of the incident happening, and are currently being held in custody in different police stations around the region.
On Friday evening there was a police presence at a caravan site near Burghfield Common, not far from where the incident took place.
Chief Constable John Campbell said PC Harper was at the scene with a fellow officer and was out of his police car when the incident occurred.
A post-mortem examination is taking place to establish the cause of death.
PC Harper was married last month in a ceremony at Ardington House in Oxfordshire and was due to go on his honeymoon next week.
Andy Fiddler, from Thames Valley Police Federation, said the news was “totally devastating”.
He said: “All our thoughts – and the thoughts of the entire police family across the UK – are with the family, friends and close colleagues of PC Andrew Harper, who died last night.
“Police officers go to work each and every day to do their duty. We are courageous, caring and compassionate. We confront danger on a daily basis.
“We know there is a risk when we put on the uniform but we do so as we are proud to protect the public.
“Sadly on very rare and horrendous occasions a colleague makes the ultimate sacrifice. We will ensure that heroism is never forgotten.”
Prime Minister Boris Johnson said he was “deeply shocked and appalled by the death of a brave officer on duty”.
He added: “My thoughts are with PC Harper’s family, friends and colleagues at this terrible time.
“It is the most powerful reminder that police officers up and down the country put themselves at risk every single day to keep us safe. They have my absolute support.”
Labour leader Jeremy Corbyn said he was “totally shocked”, adding that the incident demonstrated the need for increased focus on police numbers.
“Let’s just think for a moment of the horror of it, but also for the need for more police officers to be able to respond to disorder in our society, but also the problem of guns and the problem of violence that is growing,” he said.
The officer’s grandmother, Maureen Shrimpton, posted on Facebook: “Just a really horrible day. Our grandson Andrew was killed last night while doing his police work. So proud of him.
“Our love goes to his lovely wife, Mum and Dad, brother and all of his family and friends. Love from Nan and Bubba.”
His mother Deborah Adlam had also shared a picture with her son on Facebook – telling friends she was “so proud” of him.
Other relatives have said that the couple had a “dream wedding”.
PC Harper was also preparing to take on a huge challenge for charity – a 20-mile, 200-obstacle race – that was due to take place in May next year.
A Virgin fundraising page shows that he was hoping to raise £500 for a children’s cancer charity.
He had written: “Personally I have never been much of a runner – the longest I’ve ever done to date being 10K however I have decided to run… to try and help save the lives of children with cancer and keep families together. Even if only one child benefits then it will be well and truly worth it.
“Wherever possible, please support me in this goal and help us fight for those who can’t fight for themselves.”
The Police Roll of Honour Trust, which commends officers who have died in the line of duty, has said PC Harper’s death will be recorded on the list, adding: “His service and sacrifice will never be forgotten.”