The 22-year-old from Lifford has been missing since May 22. His family fear he has drowned
The search involving police on both sides of the border and Foyle Search and Rescue has so far concentrated on the River Mourne near Lifford Bridge.
It will be widened over the coming days, said the PSNI.
“The search will continue until Mr Colhoun is found,” police said in a statement.
David’s heart-broken dad Joe said he feared that his son drowned in the river.
In a statement issued by police Mr Colhoun thanked everyone involved in the search to date including Gardai, PSNI and volunteers from a number of organisation and the local community.
The PSNI said it has assured the family that it will continue to respond to fresh information and offer assistance and support in “difficult circumstances”.
Mr Colhoun is described as being 5ft 10in tall, of slim build with fair spiky hair and when last seen he was wearing a navy hooded top with white lettering on the front and dark tracksuit bottoms.