The General Medical Council is investigating two more doctors linked to the Hyponatraemia Inquiry.
The inquiry examined the deaths of five children in Northern Ireland.
Dr George Murnaghan and Dr Joseph Gaston were involved in the care of Adam Strain, four, who died at the Royal Belfast Hospital for Sick Children in 1995.
The GMC is also investigating a third doctor, Dr Robert Taylor, who was involved in his care.
- Who were the children?
- Timeline of hyponatraemia inquiry
Dr Taylor is already being investigated by the GMC for his role in the care of nine-year-old Claire Roberts, whose death was one of five examined by the inquiry.
In documents seen by the BBC, the GMC said it will request Adam’s medical records as part of the investigation.
The family was also asked if they had additional information they wished to add to the investigation.
The GMC said it has already received a “significant amount of information” from the public inquiry.
The BBC has asked the Belfast Health Trust and the doctors for comment.
In January, an inquiry into the deaths of the five children in hospitals found that four were avoidable.
Hyponatraemia occurs when there is a shortage of sodium in the bloodstream.
The GMC has a five-year rule whereby an investigation must take place within five years of the event having allegedly happened.
Waiving the five-year rule is significant – some other doctors named in the hyponatraemia report will not be investigated as they have either retired or the GMC’s set time has lapsed.
Speaking to BBC News NI, Adam’s mother Debra Slavin said she welcomed the development.
“This is just another stage. And each one remains difficult. We do it for Adam.
“We just hope the GMC investigation will not be prolonged and will move as quickly as possible.”
Earlier in November, it was confirmed that a doctor involved in the Hyponatraemia Inquiry would be called before a fitness to practise tribunal for the first time.
Dr Robert Quinn has been summoned by the Medical Practitioners Tribunal Service to appear in February 2019.
He was involved in the care of Lucy Crawford, who died in 2000 aged 17 months.
An investigation has to take place before a doctor can be called before a tribunal.