How a gantry crane collapsed, dropping a 90-tonne concrete slab into the Fraser River on Friday morning, is still unclear as Port Mann Bridge construction stopped for the day.

According to initial reports from WorkSafe BC, the 720-tonne yellow crane was moving a concrete bridge segment when something caused it to collapse over the bridge's new deck just after 8 a.m. Damage to the deck has not been assessed but traffic was unaffected.

There were people working near the crane when it collapsed but no one was injured.

"It was a very loud and unnatural sort of sound and I looked over at the river and there was water splashing up and I realized something very large must have gone down," said witness Edward Jordan.

Ariel Malubag was also nearby and said there was a large boom that shook his desk.

Max Logan, with the Port Mann/Highway 1 Project, said the gantry has been secured and there's no risk of further malfunction, although he does not know what caused it to collapse.

"It's too early to say what the cause of that malfunction is. There will be a full investigation to determine the cause," said Logan.

The 90-tonne slab will be pulled from the river but Logan said he is not sure when this will happen or how.

Eight lanes of ten on the new bridge are scheduled to be open by December 2012 and project managers have been pushing construction to finish, including an overnight project that saw an entire exit lane swap sides.

"At this point, it's really too early to say there would be a delay or any impact at all," Logan said. "If there is a delay, in this case, it would be the responsibility of the contractor."

With a report from CTV British Columbia's Michele Brunoro