The brother of a Bloody Sunday victim has pleaded with witnesses to the British Army killings to give police statements after detectives said there had been a disappointing response to a public appeal to assist their murder investigation.
Last month officers investigating the shootings during a civil rights march in Londonderry in 1972 urged more than 1,000 people who had given evidence to the Bloody Sunday public inquiry to contact them to make witness statements.
Thirteen people died on the day, with a badly injured man dying six months later.
Two years ago police launched a murder investigation after detectives and prosecutors reviewed the inquiry's main findings that the killings were unjustified and none of the dead posed a threat when they were shot by British paratroopers.
As evidence provided to the inquiry, which was chaired by judge Lord Saville, cannot be used for criminal prosecutions, detectives need to carry out fresh interviews with people who were there on the day, including the soldiers involved.
But the Police Service of Northern Ireland (PSNI) today conceded the response to their witness appeal had been limited.
The officer leading the investigation, Detective Chief Inspector Ian Harrison, said: "The response to our original appeal for witnesses to talk to us has been disappointing. If we are to make progress, we need witness statements."
The revelation prompted John Kelly, whose 17-year-old brother Michael was among the dead, to urge people in Derry to come forward.
"We would say certainly to come forward and help us in the final quest for justice," he said. "Justice is the final part of the equation and we would ask and plead with them to come forward and give their statements to the police.
"The police are the only ones who can prosecute these guys."
The conclusions in Lord Saville's 12-year public inquiry prompted Prime Minister David Cameron to apologise to the relatives of the victims.
The probe contradicted the long-standing official version of events, outlined in the contentious 1972 Widgery report, which had exonerated soldiers of any blame.
The shootings took place in the nationalist Bogside area of the city, where resentment and distrust toward the military and police were prevalent during the Troubles.
Mr Kelly said people had no need to fear contacting the police now.
"I think it boils down to the fact that the people are not sure at this moment in time what they should be doing," he said.
"We have put out an appeal for people to come forward and if people aren't sure, contact us, talk to a family member (of one of the victims)."
He added: "There is no fear in talking to the police and we would support what the police are doing in trying to bring people in to give statements. They shouldn't be reluctant, they should be quite prepared to do so because there is no fear attached to it.
"This is about what happened 42 years ago and they are the people who can help, they are the only people can help in the final part of our quest for justice."
Mr Harrison said he was determined to conduct a thorough, professional and effective investigation.
He assured anyone who engaged with the detectives that all matters would be treated in confidence and support would be offered to witnesses.
"We are renewing our appeal and placing additional advertising to increase awareness of what we're working to achieve," he said.
"The adverts asking people to come forward will be in local newspapers and on billboards in Derry.
"We need people to work with us. If people don't come forward, it will further delay this lengthy and complicated process."
Police asked civilians and former soldiers to contact the investigation team on 028 9025 9593 or by email to BloodySundayEnquiry@psni.pnn.police.uk