More than 1,000 witnesses, including the public and former soldiers, are being asked to contact police investigating the events in Londonderry in January 1972.Thirteen people died when British Army Paratroopers opened fire on a civil rights march taking place through the Bogside area of the city.A 12-year long Inquiry conducted by Lord Saville found that there was no justification for the shootings and, subsequently, Prime Minister David Cameron apologised for the killings.Following the inquiry the PSNI announced it was to conduct an investigation into the incident.At the beginning of this year investigating officers issued an appeal for witnesses and those that gave evidence to the Saville Inquiry to contact them.Police are unable to use testimony from the Saville Inquiry as part of their criminal investigation.On Thursday, police revealed there had been a "limited response" from the public and reissued their appeal for those with information to come forward.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."We are renewing our appeal and placing additional advertising to increase awareness of what we're working to achieve."Adverts asking people to come forward will be in local newspapers and on billboards in Derry.""If we don't get prosecution, then I feel the whole thing will just be lost and it will mean those responsible for murder and mayhem on Bloody Sunday will not be brought to book."Mickey McKinney, victim's brotherPolice would not reveal how many people had contacted them following the initial appeal.Inspector Harrison added: "We need people to work with us. If people don't come forward, it will further delay this lengthy and complicated process."Police have also assured all who engage with the investigation team that all matters will be treated in the strictest confidence."The support and welfare of witnesses are important considerations and it is our intention to conduct these inquiries as quickly as possible," added a PSNI spokesman."The police service is determined to conduct a thorough, professional and effective investigation."Mickey McKinney, whose brother William was killed in Bloody Sunday, said it was vitally important people contacted police.He said: "It's important any and all information is passed along to the police."It's very important that the police have enough information to pass along to the Public Prosecution Service to get this into court."