Sinn Féin has accused the PSNI of manipulating the investigation into the murder of Robert McCartney in order to damage the party.
Police were also accused by a republican source of refusing to interview a key suspect who made himself available to them yesterday. The eyewitness account would have allowed immediate arrests and charges to be made, the source told The Irish Times.
Sinn Féin's chief negotiator, Martin McGuinness, said the campaign to bring the killers to justice could be left open to political manipulation.
"The McCartneys need to be very careful. To step over that line, which is a very important line, into the world of party political politics can do a huge disservice to their campaign," he said.
"In fact, it can dismay and disillusion an awful lot of people, tens of thousands of people who support them in their just demands."
Mr McGuinness and a senior republican source last night claimed that the PSNI was "tailoring" its investigation to cause maximum damage to Sinn Féin - a charge rejected by the police last night.
The republican source said a key witness to the murder of Robert McCartney went to the PSNI yesterday to make a signed eyewitness statement, but was "sent away" because the senior investigating detective was not available.
This witness had already given a taped statement to the PSNI naming "a number of those involved in events around the murder of Robert McCartney", according to the republican source.
The statement that this witness wished to give yesterday "would have allowed immediate arrests and charges to be brought", the source said.
The source named three people who, he said, were witnesses to the events. One person who was with Mr McCartney in and outside Magennis's bar the night he was murdered gave a statement to the PSNI naming a person who assaulted him (this witness), he said.
A second witness, who was with Mr McCartney in the bar, gave a detailed statement to the police naming those involved "in the fracas in Magennis's", the source added.
"None of the people named by any of the three key witnesses has been charged by the PSNI," he said.
The fourth person available to give a statement was one of the key suspects in the murder, the republican source said. He went to his solicitor yesterday "with the intention of presenting himself to the PSNI", he added.
His solicitor contacted the police twice over several hours to say his client was available for interview. Police eventually told him that the "senior investigating detective was away at a conference and police did not have the manpower to deal with his client, and that he should call back in a couple of days", the source said. Based on this response from the PSNI, it was clear that police were "less than enthusiastic" about charging those who at a minimum had "significant eyewitness testimony", he added.
"The only logic of this is to increase the frustration of the McCartney family because this frustration is being directed at Sinn Féin," the source concluded.
Last night, Mr McGuinness said that there was growing evidence that the McCartney family's campaign for justice was being exploited and manipulated by the PSNI to damage Sinn Féin.
"Republicans have suspected that the PSNI is tailoring their investigation into the murder of Robert McCartney to cause maximum damage to Sinn Féin. Today's revelations that the PSNI turned away a key witness and a key suspect adds further weight to this suspicion," he said.
"In such a high-profile murder investigation, it beggars belief that the PSNI would reject the offer of an eyewitness statement from a key witness and the opportunity to interview a key suspect who they claim to have been searching for," added Mr McGuinness.
The PSNI said its sole purpose was in gaining justice for the McCartney family.
"Police refute the distractions which have been peddled. Our sole interest is to bring justice to the killers of Robert McCartney and bring closure to the family," said a spokeswoman.