An excavation has begun on land near where six-year-old Mary Boyle disappeared on March 18 1977.
Gardai confirmed the dig was taking place in Co Donegal, understood to be on bogland near Cashelard, a few miles from Ballyshannon.
Mary disappeared while visiting her grandparents’ home a few miles from the town 39 years ago.
Her twin sister Ann Doherty intensified a campaign over the last year over the handling of the original Garda investigation and her lawyers wrote to the force during the week over reports that the case had been handed over to the cold case detective unit.
Ms Doherty is also understood to be lodging a complaint with the Garda Ombudsman over the development being reported in the media without her being notified.
The Garda press office said the Mary Boyle disappearance is under active investigation and that it will continue.
It said there have been two reviews of the case. The latest started in 2011 and included interviews with “a wide range of people” and “a number of searches with the assistance of forensic and geology experts”.
One man was arrested and released without charge.
It also confirmed the cold case unit – the Serious Crime Review Team (SCRT) headed by Detective Superintendent Walter O’Sullivan – was recently asked to examine the case.
“The primary purpose of a review is to assist senior investigation officers who are investigating a serious crime by identifying new and potential investigative opportunities. Members of the SCRT are trained in homicide investigation and in the reviewing of unresolved homicides,” it said in a statement.
A march aimed at raising awareness of the case is taking place in Ballyshannon on Saturday.
Mary was from Belcruit, Kincasslagh, in the north of Donegal and went missing while on a St Patrick’s Day visit to relatives with her parents Ann and Charlie and siblings.
Her body has never been found and her disappearance remains the longest running case of its kind in Ireland.
Repeated searches have taken place in bogland and fields around Cashelard over the years.
Mary had been playing outside when she followed her uncle across fields near her grandparents’ house but was never seen after she turned to go back.