FAMILIES of the Clutha helicopter disaster victims' have been left angry after the official report into the disaster gave no clear explanation for the crash.

There were heated words between some of the 10 victims and senior air accident investigators as the 102 page document was released to families at a private briefing tonight.

John McGarrigle, who lost his father said: "we have waited two years for this but I'm furious. People will understand when they see the report on Friday. I've been left with more questions than answers."

One relative said the Air Accident Investigation Branch (AAIB) had offered them no explanation for the accident other that that it was "some sort of technical malfunction".

Ian O'Prey, whose son Mark died when a Police Scotland helicopter crashed into the Clutha Vaults on November 29 2013, said: "They can't explain why the switches were off. There's nobody to blame.

"It was heavy with technical jargon. It's been three hours of that and my head was burning with it."

He claimed the probe conclusions hinged on switches which control the flow of fuel from the tanks to the engines.

These had been found in the off position by investigators when they should have been turned on, he added.

Mr O'Prey admitted "heated words" had been exchanged by different groups of relatives during the meeting, but that he felt relieved the investigation was now complete.

Dave Bell, a lawyer for Irwin Mitchell solicitors which represents many of the families, pilot error could not be ruled out.

He added that if the helicopter had been fitted with a black box "many of the unknowns would be known."

Colleague Jim Morris said that it was "incredibly frustrating" that the report did not give a clear explanation for what caused the crash.

He added: "there are a number of safety recommendations in the report which cannot be revealed at present due to confidentiality reasons.

"Human factors were examined in detail but there are still a lot of unanswered questions in relation to the human factors."

Mr Morris said the lack of clear answers was "incredibly frustrating" but he added: "The AAIB have done their best and gone to incredible lengths to produce this report."

The document was released to the families at a three hour meeting at Glasgow's Marriott hotel. It comes ahead of the report's official publication tomorrow.

A second briefing is planned there this morning for people who were unable to attend.

However, the AAIB have come under fire for their handling of the report's disclosure, with lawyers for the victims, Police Scotland and even those severely injured in the crash kept in the dark.

Only the families of those killed in the tragedy were sent letters inviting them to attend the briefings.

Lawyers acting for the families were barred entry, while people who sustained severe and life-changing injuries have been left furious after they were excluded from the meetings.

The AAIB has infuriated legal teams further by refusing to explain the rationale behind its decision to invite some victims but not others.

Senior officers at Police Scotland, which lost two police constables in the disaster, are also understood to be angry after receiving no communication from the AAIB about its intention to hold the family briefings or its plans to publish the report on Friday.

Until now, lawyers for the key stakeholders - including Police Scotland, operator Bond, and manufacturer Airbus Helicopters - have only had sight of a draft version of the report.

They will see the final version for the first time on Friday, nearly two years after the crash.

Pilot Captain David Traill died along with PC Kirsty Nelis and PC Tony Collins after the police helicopter, a Eurocopter EC135, plunged into the roof of the Clutha Vaults pub at 10.22pm on November 29, 2013, as it was returning from routine surveillance.

Mr O'Prey, along with John McGarrigle, Gary Arthur, Colin Gibson, Robert Jenkins, and Samuel McGhee, who were inside the pub, were killed when the roof caved in. Joe Cusker, who was also at the venue, died from his injuries in hospital 13 days later.

The latest criticism of the AAIB comes after First Minister Nicola Sturgeon complained about delays in investigators publishing their findings.

She urged Transport Secretary Patrick McLoughlin to "take any necessary steps to expedite its publication", which was originally scheduled for mid-2015.

The AAIB have not published any details of their investigation since an interim report in February 2014, which said that both of the helicopter's engines had "flamed out" within seconds of one another due to fuel starvation despite 76kg of fuel remaining in the main tank.

Investigators found that switches which control the flow of fuel from the tanks to the engines were switched off, when they should have been on throughout the flight.

There was no mayday and nothing to indicate that the pilot had attempted an emergency landing.

The inquiry has also been complicated by the lack of a black box on board the aircraft.