The family of a Catholic man murdered by a sectarian mob appealed today for a public inquiry to uncover the truth after a litany of allegations were levelled against police over the case.

Robert Hamill, 25, was attacked in Portadown Co Armagh in 1997, during a night out with friends, but armed police at the scene were accused of failing to protect him and frustrating the subsequent investigation.

No one has been convicted of murdering Mr Hamill, but today the opening session of the inquiry heard that within days of the killing police had names for those involved, plus evidence a Royal Ulster Constabulary (RUC) officer warned one of the killers to dispose of clothes worn during the attack and updated him on the investigation.

"We are very hopeful we are going to get to the truth," said the murdered man's sister Diane, whose family attended today's hearings.

"If there were any mistakes made, we want those to be acknowledged, but if there were any deliberate actions taken to allow the people that murdered my brother to walk free, if anyone helped them, we need that to be exposed."

The Hamill murder was one of the most high profile of the Troubles, with the sectarian killing compared to the racist murder of black teenager Stephen Lawrence and the handling of that case by the Metropolitan police.

The controversy grew after the Hamill family's solicitor, Rosemary Nelson, who lobbied on the case, was murdered in a loyalist bomb attack amid allegations of security force collusion.

Mr Hamill's inquiry opened in Belfast's Interpoint Centre, where a public inquiry into Mrs Nelson's death is already being held.

Ashley Underwood QC, lead counsel for the Hamill Inquiry, outlined the case and recounted claims Mr Hamill was attacked on April 27, 1997, while an RUC Land Rover carrying four armed officers was parked nearby.

He revealed: "By May 10, 1997, the RUC had the identities of a number of Protestants who were said to have murdered Mr Hamill.

"Further, it had evidence that one of the reserve constables in the Land Rover, Mr (Robert) Atkinson, had protected one of them, by telling him to get rid of his clothing and by keeping him updated about the investigation.

"However no-one has been convicted of murdering Mr Hamill and only one person was convicted of affray arising out of the attack on him.

"Reserve constable Atkinson was eventually charged in relation to a conspiracy arising out of the alleged tip-offs that he gave, but was not prosecuted to trial."

Margaret Ann Dinsmore QC, legal representative for Mr Atkinson, today said he and his wife Eleanor denied all allegations of wrongdoing.

Lawyers for other police officers today also denied the allegations and called on the inquiry to separate fact from rumour.

Martin Woolfe, junior counsel for the Police Service, pledged its cooperation with the inquiry and denied Mr Hamill's case was treated differently because he was a Catholic.

Mr Hamill was a father of three, with two sons, Shane, 17, and Ryan, 15, while his 11-year-old daughter Nicole was born after he died.

He was attacked in Portadown, a sharply divided town where religious tensions between the majority Protestant population and minority Catholic population have often led to violence.

The area came to wider attention as a result of the Drumcree marching dispute in the mid-1990s when nationalists opposed Orange Order parades through the Catholic Garvaghy Road community.

Mr Hamill suffered serious head injuries and died 11 days later in hospital without regaining consciousness.

And while an inquest has never been held, a post mortem examination showed the assault was the cause of death.

In 2004 the government set up a public inquiry after a review of the case by retired Canadian judge Peter Cory who examined a number of murders where security force involvement was alleged.

He recommended inquiries into the loyalist murder of solicitor Pat Finucane, the republican murder of loyalist Billy Wright in the Maze prison, the IRA murder of RUC officers Harry Breen and Robert Buchanan, plus the murders of Mr Hamill and Mrs Nelson.

Six men were charged with murder over the Hamill case but charges against five were dropped, while a sixth man was sentenced for affray.

The Director of Public Prosecutions decided not to prosecute the police officers on duty on the night of the fatal assault.

Two people were sentenced after admitting giving false information about a phone call to a suspect's house.

The admission resulted in charges of perverting the course of justice being brought against RUC reservist Robert Atkinson, his wife Eleanor and a third person.

Charges against the trio were dropped after a key witness was ruled unreliable.

Mr Underwood today catalogued concerns over how police behaved on the night Mr Hamill was attacked, how the murder investigation was handled, how the allegations against Constable Atkinson were pursued, as well as the decisions of the Prosecution Service.

The inquiry has the power to compel witnesses, while evidence given to the inquiry cannot be used to secure convictions.

The Hamill probe will hear from 160 witnesses - including the police officers at the centre of the allegations - with a final report expected by the middle of next year.

The probe has so far cost £18.8 million, but officials estimate the final bill will be £36 million.

The Robert Hamill Inquiry will be chaired by former English High Court judge Sir Edwin Jowitt, who will be assisted by former chief constable of Devon and Cornwall Constabulary Sir John Evans and moderator of the Churches' Commission for Inter Faith Relations the Rev Kathleen Richardson, Baroness Richardson of Calow.

The Hamill family will be represented by Barra McGrory QC, while the Police Service of Northern Ireland will be represented by Richard Ferguson QC.

Sir Edwin today said: "It is our hope that our work in this inquiry may do a little to help people (in Northern Ireland) to look forward with hope."