Two women have been arrested by detectives investigating the murder of Pc Dave Phillips, who was mown down and killed by a stolen car.
The pair, aged 19 and 34, from Wallasey, Merseyside, were detained on Tuesday night on suspicion of assisting an offender, police said.
Their arrests come after two men, aged 18 and 30, from the Oxton and Wallasey areas of the Wirral, were held in connection with the killing of Pc Phillips in the early hours of Monday.
The women are being questioned at police stations in Merseyside.
On Tuesday, Merseyside Police said officers had executed four warrants as part of the murder investigation into Pc Phillips’s death as detectives continued to appeal for information.
The 34-year-old father of two died after he was sent to assist following a burglary at an estate agent in Birkenhead, where a red Mitsubishi pick-up was stolen just after 1am on Monday.
Following a police pursuit at about 2.08am, Pc Phillips and a colleague attempted to use a “stinger” device to stop the Mitsubishi Challenger L200.
The vehicle mounted a central reservation and headed directly at the officer before speeding off. Pc Phillips was given CPR by colleagues at the scene but died a short time later at Arrowe Park Hospital on the Wirral. The pick-up was later found abandoned in Corbyn Street, Wallasey.
A memorial fund set up in Pc Phillips’s memory has so far raised over £45,000 from more than 2,000 donors.
Merseyside Police Federation Charitable Trust set up the Just Giving page to honour an officer “tragically and unlawfully killed whilst on duty”.
Detective Chief Superintendent Paul Richardson thanked the public for its support.
He said: “Since the investigation was launched yesterday we have been inundated with calls from members of the public and as a result we are following up a number of positive lines of inquiry.
“From the hundreds of supporting and caring comments on social media and media websites yesterday, it is obvious that there is overwhelming support for the police and that the public appreciate the work undertaken by officers like Pc Phillips to keep their communities safe.
“I would like to appeal to any members of the public who have any information which could help us with the investigation to come forward.”