Two men have been arrested after a policewoman died in a collision between an unmarked police car and a 4x4 vehicle in Derry.
The officer has been named as 27-year-old Philippa Reynolds, who was from the greater Belfast area.
She was a back seat passenger in an unmarked police car that was in a collision with the 4x4 on the Limavady Road, at around 3.30am.
The men, aged 23 and 25, were arrested this afternoon.
The PSNI has appealed to any witnesses to come forward.
Constable Reynolds, a former teacher, had been in the Police Service of Northern Ireland for two years.
Her commander in Derry, Chief Superintendent Stephen Cargin, paid tribute to her.
"She was enthusiastic, bubbly and had a beautiful personality," he said.
"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.
Chief Superintendent 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 sustained non-serious injuries.
He said it could have been any vehicle turning the corner into the path of the 4x4.
Northern Ireland Chief Constable Matt Baggott has vowed to bring to justice those responsible for the car crash.
He said he was greatly saddened by the death of his colleague.
"This is a personal tragedy for her family and also for the whole Police Service," he said.
He continued: "She was out on duty, working to keep people safe, when she died. My thoughts and prayers are with her family at this time.
"I, and my colleagues, will ensure that no stone remains unturned as we work to bring to justice those who perpetrated this crime".
Stormont Justice Minister David Ford said his thoughts were with the policewoman's relatives and the injured officers.
"I was shocked to hear of this tragic incident which resulted in the death of a police constable who was carrying out her duties serving the community in the middle of the night," he said.
"My thoughts and prayers are with her family at this time.
"I also send my best wishes to the two officers injured in this incident for a speedy and full recovery. It is hard to imagine how they, and indeed the wider PSNI family, are feeling.
"This is not the first time we have seen death and injury resulting from car crime. I call on those engaged in this crime to reflect on the consequences of their actions and stop this senseless activity."
Stormont First Minister Peter Robinson and Deputy First Minister Martin McGuinness also expressed sympathy at the officer's death.
In a joint statement, the ministers said: "This is devastating news and we want to extend our heartfelt sympathies to the Police Officer's family, friends and colleagues.
"We wish her two police officer colleagues a speedy and full recovery.
"Officers in the PSNI put their lives at risk on a daily basis to serve and protect our community and we are all truly grateful for their dedicated service.
"Those who commit a crime such as vehicle theft must realise their actions can have serious consequences. We urge anyone with information to go to the police."
The Northern Ireland Police Ombudsman's office has been informed.
Garda Commissioner Martin Callinan extended his condolences.
"Our thoughts and sympathies are with our friends in Northern Ireland as they mourn the loss of their colleague," he said.
"We also wish a speedy recovery to the officers injured in the incident."