Constable Stephen Carroll was fatally wounded when shots were fired through a rear window of his police car.
The shooting happened at about 2145 GMT near Lismore Manor in Craigavon, as police were responding to a call for help from a woman.
Shortly after the incident, police sealed off roads surrounding the area as an operation to catch the killers commenced.
As daylight broke, forensic staff started their investigation, carrying out a finger-tip search of the area.
The shooting took place close to Lismore Comprehensive school, which is situated beside Lismore Manor.
The car in which Constable Carroll was fatally wounded is covered with a tarpaulin and towed away by forensic experts for more detailed examination.
There is still a heavy police presence in the area, with many officers armed, amid fears of further attacks.