Detectives in the North investigating the disappearances of missing schoolgirl Arlene Arkinson were at a disadvantage because of the delay in reporting her missing, an inquest has heard.
Brian Lavery, a former Police Service of Northern Ireland (PSNI) detective sergeant who reviewed the investigation papers for the National Crime Faculty, said there is only a small window of opportunity to gather evidence in such cases.
He said: "Once that time elapses then unfortunately you are playing catch up."
Mr Lavery was giving evidence at Belfast Coroner's Court for a second day.
Fifteen-year-old Arlene from Castlederg in Co Tyrone vanished after a night out across the border in Co Donegal in August 1994. She was last seen being driven off late at night with convicted child killer Robert Howard on August 14 1994.
However police did not receive a missing persons report until four days later, the court was told.
Kevin Rooney QC, barrister for the PSNI, said police had also been told lies by eye witnesses and that Howard had tried to throw them off the scent with a fake sighting.
The lawyer also revealed that in 1999 research showed some 96% of murdered children were killed within the first 24 hours and that 76% were actually killed within six hours of the last sighting.
The chances of a successful prosecution were also significantly reduced if there was no arrest within 48 hours and if a body was not recovered, he added.
"If that's right it looks like the report of a missing person is vital within 24 hours," said Mr Rooney during cross examination.
Mr Lavery agreed: "Absolutely, yes."
Despite the odds, Mr Lavery recommended police continue with the case against Howard adding that in 1999 he knew of eight successful prosecutions in similar circumstances.
"If it is unsolved does not mean it is unsolvable," he said.
Howard, 71, who died in prison last year, was acquitted of the Arlene's murder in 2005 by a jury which was not told of his previous conviction for killing a South London teenager several years earlier.
The hearing has been adjourned until tomorrow.