Two men are being questioned in connection with a car crash, which killed a "bubbly and beautiful" policewoman in her patrol car.
Pc Philippa Reynolds, 27, from the Greater Belfast area of Northern Ireland, died when the unmarked car she was travelling in turned into the path of a stolen speeding 4x4, which had just run a red light, in Londonderry.
Two men, aged 23 and 25, have been arrested following the incident on the Limavady Road at around 3.40am on Saturday.
Ms Reynolds' commander in Derry said he and fellow officers were devastated at the loss of a colleague they all loved. Ms Reynolds, a former teacher, had been in the Police Service of Northern Ireland for two years.
Chief Superintendent Stephen Cargin, said: "She was enthusiastic, bubbly and had a beautiful personality. She was just loved by her colleagues and she turned that love to the community she served."
The patrol car was turning a corner when the stolen silver Toyota Land Cruiser hit it side on. Mr Cargin said he believed the officer, who was sitting in the rear driver's side seat, would have died instantly due to the force of the impact. Two other officers in the car suffered non-serious injuries.
The commander said it could have been any vehicle turning the corner into the path of the 4x4. "It could have been anyone, it could have been a family that was struck," he said.
"She was a young 27-year-old officer, just two years' service, in the prime of her life, doing what she loved to do - and that was serving the community to the best of her ability," he said. "Her colleagues are devastated, as I am as her commander, as we all are as a police service."
Northern Ireland's chief constable has vowed to bring to justice those responsible. Matt Baggott said: "I and my colleagues will ensure that no stone remains unturned as we work to bring to justice those who perpetrated this crime."
Police had been hunting for two men who made off from the scene in the direction of the Ebrington area. The Toyota was stolen in the Gobnascale area of Derry around 3am.
Mr Cargin appealed to anyone who had seen the vehicle from that time, or who witnessed the crash, to come forward. The Northern Ireland Police Ombudsman's office has been informed.