The Bloody Sunday Inquiry is to reconvene to hear the evidence from a man known only as Witness X.
The operation at the Guildhall has been dismantled
He denies telling the police in 1972 that he fired two magazines from a rifle on Bloody Sunday.
The inquiry has been investigating the deaths of 14 civilians shot by soldiers during a civil rights march in Londonderry in January 1972.
It was thought the tribunal had finally ended last month, after seven years and at a cost of about £150m.
Bloody Sunday inquiry facts
Lord Saville held his first hearing at Derry's Guildhall in April 1998.
The inquiry began to hold public hearings in March 2000
The tribunal has now sat for 433 days.
It has heard evidence from 921 witnesses.
There have been 1,555 written statements from witnesses.
The final bill will be around £150m.
The final report is expected next summer.
The whole operation at the Guildhall in Derry, where much of the tribunal took place, has been dismantled including computers and video screens.
It is understood that computer hard drives have been destroyed because of the sensitive information held on them.
It involves bringing back the three judges, various legal teams and staff to hear this one witness.
Witness X is alleged to have told the police in 1972 that he was a member of the Provisional IRA and that he fired a gun from Glenfada Park.
Many of the people were killed in this area, and the soldiers said they saw gunmen there.
Witness X denies this and says he has never been a member of the IRA.
A subpoena was issued last January for him to give evidence, but he did not attend, giving medical reasons as his excuse.
If he does give evidence this time, he will be anonymous, behind screens and may appear via a video link.
Soldiers shot 13 people dead in Derry on Bloody Sunday
After hearing from more than 900 witnesses, the inquiry was thought to have finished hearing evidence when Lord Saville and his two colleagues retired to write their final report last month.
The Bloody Sunday inquiry was established in 1998 by Prime Minister Tony Blair after a campaign by families of those killed and injured.
Lord Saville of Newdigate and the Commonwealth judges accompanying him on the inquiry began hearing evidence in March 2000.
The inquiry has heard evidence from leading politicians, including the prime minister at the time, Sir Edward Heath, civilians, policemen, soldiers and IRA members.
Lord Saville's final report and conclusions are not expected to be made public until next summer.