A four-piece work platform can be secure, but it has a greater probability of failure than a two-piece scaffold, a safety expert says. (CBC)

The investigation into a construction accident that killed four men on Christmas Eve at a Toronto apartment building has elicited concerns about the safety of the platform that the workers were standing on before it broke in two.

Five men were repairing balconies on the 13th floor of the 18-storey building when the suspended platform, known as a swing stage, split and they plunged to the ground. Four of the men died at the scene and a fifth was taken to hospital in critical condition.

Questions are being raised about the safety of the platform, because it came in four pieces instead of the usual two.

"The more connection points you have with any mechanical system, the greater the probability of failure," said Rob McLellan, health and safety expert with the Clarkson Group, a Mississauga, Ont.-based consulting firm.

"So if you have two connection points, that gives you one probability of failure. If you double that, your probability goes up. If you triple that, it goes up even higher.

"It's not that they're unsafe, but they have to be designed and maintained ... to maintain their safety."

The Ontario Labour Ministry and Toronto police are investigating Thursday's accident, which occurred near Kipling Avenue and Steeles Avenue West. So far, neither authority has said whether the men's safety harnesses were attached when the swing stage snapped.

But McLellan said that's a secondary consideration because "protection equipment is your last resort."

"There had to have been a whole bunch of other things gone wrong before the fall protection equipment should have even come into play. If they were not wearing it, if they were not tied off, that's wrong, it's against the regulations. But… there had to have been other things that went seriously wrong on that scaffold for those people to have fallen."

Police have said the victims were of European backgrounds, but they won't release identifying information until their families can be notified.