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.