An eyewitness described how the crane "toppled over"

Michael Whittard, 36, from Leeds, Martin Burgess, 31, from Castleford, West Yorkshire, and Peter Clark, 33, from Southwark, south London, died when the crane they were working on crashed to the ground.

It was being used to construct the HSBC skyscraper in London's Canary Wharf when the top snapped off on 21 May 2000

The jury at St Pancras Coroner's Court in central London, returned an open verdict on Thursday.

Workers Eamonn Glover and Gareth Hetherington survived the accident by hanging on to the remaining mast and running down 27 flights of the building, which was then just a shell.

The Health and Safety Executive said that despite extensive investigation, it accepted the precise technical cause may never be established conclusively.

"It is very disappointing for us and the families that it has not yet been possible to pin down exactly what caused the accident," a spokesman said.

I cannot understand when three people get killed and no one is to blame Tony O'Brien, Construction Safety Campaign

They had almost finished when part of the crane, called the climbing frame, began twisting.

Mr Glover told the inquest he shut his eyes as "the whole thing shook".

"I actually thought I was going down with the crane," he said.

"When everything stopped I looked up and there was no crane there."

'Could have been higher casualties'

After the accident the Health and Safety Executive (HSE) published an industry-wide discussion paper about how to improve the operation to lengthen cranes.

The inquest heard a special safety plug was missing and there was no anemometer in the crane cab to measure wind speed, although there were no weather problems that day.

Tony O'Brien, national secretary of the organisation Construction Safety Campaign said: "I cannot understand when three people get killed and no one is to blame."

"It happened on a Sunday. If it had happened on a weekday it could have been worse.

"I don't want to think about the amount of people that could have been walking around there."