By Paul McCauley
BBC Northern Ireland correspondent
Lord Saville's inquiry moved to London in 2002 over security fears
The Bloody Sunday Inquiry has resumed in London.
The inquiry is investigating the deaths of 13 civilians killed when members of the Parachute Regiment opened fire during a civil rights march in Londonderry in January 1972. A 14th person died later.
It is the longest and most expensive public inquiry in British legal history.
It was set up in 1998 by Prime Minister Tony Blair and began hearing evidence in Derry three and a half years ago.
However, the tribunal moved to London in 2002 as the soldiers refused to testify in Derry because of fears for their safety.
So far the tribunal, chaired by Lord Saville of Newdigate in the company of two Commonwealth judges, has cost around £115m with the cost expected to rise to around £160m.
However, the end is now in sight.
The tribunal will sit at Central Hall in London for about six weeks before returning to Derry to hear the remainder of the evidence.
The tribunal hopes to finish all of the evidence by Christmas.
The soldiers deny civilians were ever targeted
Lord Saville has heard evidence from nearly 800 witnesses.
They include the prime minister in 1972, Sir Edward Heath, and the current Chief of the General Staff, General Sir Mike Jackson, who was a captain in the paras on Bloody Sunday.
Many civilian witnesses have told the tribunal the paras opened fire indiscriminately when they entered the Bogside area of Derry and people were shot without justification.
The soldiers have insisted they only fired at gunmen, nail bombers and petrol bombers.
However, some key witnesses will be giving evidence over the next few months.
Soldier H fired 22 shots on Bloody Sunday, claiming that 19 of the bullets were fired at a gunman at a window in the Bogside.
When the first Inquiry into Bloody Sunday was held in 1972, the Lord Chief Justice at the time, Lord Widgery, said Soldier H did not fire 19 shots at a gunman and therefore those bullets were wholly unaccounted for.
Soldier F fired 13 shots on Bloody Sunday and the forensic evidence from the time indicated that he killed one of the deceased, 17-year old Michael Kelly.
Soldier F claimed he fired at a man about to throw a nail bomb but Lord Widgery said he was not satisfied that Mr Kelly was throwing a bomb when he was shot.
Both soldiers will be in front of Lord Saville within the next couple of weeks.
Sinn Fein MP Martin McGuinness is expected to be the first witness to testify when the tribunal returns to Derry.
He will tell the tribunal he was the Provisional IRA's second-in-command in Derry on Bloody Sunday and that the IRA members obeyed his orders not to shoot at the security forces during the civil rights march.
At least five former members of the Official IRA will also give evidence in Derry.
They will tell the tribunal that they fired a small number of shots but only after the paras had already shot a number of civilians.
Mr McGuiness will say he ordered IRA members not to shoot
If the tribunal finishes hearing evidence by Christmas it will still have to sit again to deal with closing submissions and any outstanding matters.
The final report is likely to be published in the autumn of next year.