Complaint: Catherine, Claire and Paula McCartney

The McCartney sisters have expressed serious concerns about how the North's police ombudsman is handling a complaint they made about the PSNI investigation into the IRA murder of their brother Robert.

Catherine McCartney said: "We've been through the motions of a trial that was never going anywhere. If there's a similar charade from the ombudsman's office, then we won't go along with it.

"If we aren't satisfied that the ombudsman is investigating every aspect of our complaint against the PSNI, we will withdraw that complaint and pull out of the process."

Father-of-two Robert McCartney (33) was beaten and stabbed to death by the Provisionals outside Magennis's bar in Belfast city centre in January 2005. Three men were later charged in connection with the killing.

Terence Davison was charged with murder; James McCormick and Joseph Fitzpatrick with causing an affray. The trio were acquitted in June 2008. The following year, the McCartney sisters lodged a complaint about the PSNI investigation with the ombudsman, whose report is expected this summer.

Catherine McCartney said: "The PSNI case was so weak, there shouldn't have been a trial in the first place."

The sisters suspect that during the police investigation, British agents in the IRA were protected.

"We don't believe the PSNI pursued the cover-up after Robert's murder in terms of those who cleaned up Magennis's bar and hid evidence and those who witnessed Robert's beating and stabbing. Up to 12 people saw what happen­ed yet no one was charged with withholding information." McCartney said she feared the ombudsman's office wasn't adequately investigating the PSNI's response to the IRA's cover-up of the murder.

"We are on the verge of losing confidence with the ombudsman's office. If we feel they aren't doing their job thoroughly, we'll withdraw rather than waste our efforts," McCartney said.