Two men remain in custody this morning in connection with the shooting of journalist Lyra McKee in the Creggan area of Derry on Thursday night.
The PSNI is concerned the person who shot dead the 29-year-old is a new breed of terrorist.
Detective superintendent Jason Murphy said terrorists were lurking in the shadows.
Both who are being held are aged 18 and 19 and are being questioned under Northern Ireland's terrorist act.
The PSNI issued footage of the masked shooter who shot into a crowd of police and civilians.
Detective Superintendent Jason Murphy, who is leading the investigation into the murder of 29-year-old Lyra McKee in Derry/Londonderry tells media of a ‘palpable sea change in the sentiment towards community policing’. pic.twitter.com/g7WKcReYzC— PSNI (@PoliceServiceNI) April 20, 2019
Detective Superintendent Jason Murphy also made an appeal for those who had mobile phones in the area.
"I know that a large number of people were in the area of Fanad Drive and Central Drive on Thursday night.
"I would ask those individuals to please come and speak to my detectives with regards the events of Thursday night and to allow us to examine the video footage on their mobile phones."