Newlywed PC Harper was killed while responding to reports of a burglary in the Berkshire village of Bradfield Southend at around 11.30pm on August 15.
The 28-year-old constable died from multiple injuries after being dragged under a vehicle and possibly hit by a police car.
On Monday, Thames Valley Police said investigators had been granted another 24 hours from Sunday night to question 10 suspects over Pc Harper's death.
The group, males aged between 13 and 30, were all arrested at a caravan park within an hour of the incident.
Donations have poured in to support PC Harper's bereaved family, with a Just Giving appeal by fellow officers reaching six figures within the first day.
The fundraising page set up by Thames Valley Police Federation to support the officer's widow, Lissie, and his family broke the £100,000 mark on Sunday night with donations continuing to pour in.
It reads: "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."
Flowers have been laid at the scene where the young constable lost his life.
One tribute reads: "A great cop, a great man, a great friend. There will always be a part of my heart missing now."