An official with Canadian Pacific has said the massive flooding experienced in Calgary was behind a bridge failure that caused six train cars to derail over the Bow River early Thursday morning.

Mark Seland, a spokesperson with CP, said the sudden influx of water caused one of the bridge’s four piers to scour, which then caused the bridge to sag.

“Because the scouring was at the bottom, it made it impossible to actually inspect and detect this problem,” Seland said during a news conference Thursday afternoon.

Seland said the bridge has been inspected 18 times since the flooding began. It’s normally inspected twice weekly, as required by federal regulation.

Earlier in the day Calgary Mayor Naheed Nenshi slammed CP after the bridge failed.

Shortly after the accident, CP said the bridge was last inspected on Saturday and the tracks were inspected on Monday.

But Nenshi said Thursday that the bridge should have been inspected yet again, and suggested that recent layoffs at CP may have led to a decrease in bridge inspectors.

However, Seland said the number of bridge inspectors has not changed.

“We have the same number of bridge inspectors today that we had prior to any layoffs,” he said. “It’s the same number we had a year ago and five years ago.”

CP officials could not say how many inspectors typically examine the bridge in question, but said there are usually two to three inspectors at each division.

Five of the derailed cars were carrying empty petroleum desolate, which is a material found in paint thinner, metal polish and wood stain. The sixth derailed car was empty.

Seland said all the derailed cars are upright, and no leakages or injuries have been reported.

The train is expected to be removed from the bridge sometime early tomorrow morning, at which time CP will decide what to do with the bridge.

Nenshi had earlier said that railways are private companies, which has been a point of contention for municipalities that are powerless to regulate them.

"I will tell you that this has been a constant frustration for every municipal politician in this country forever," the mayor said during a press conference.

He assured the public that municipal bridges in Calgary are safe, and have been inspected three times since the flood.

The mayor said the bridge in question is old and was not built into the bedrock -- "something I didn't know until today" -- whereas all the city's bridges are.

“We have to have a serious conversation about this,” he said. “How is it that we don’t have regulatory authority over this, but it’s my guys down there risking their lives trying to fix it?”

He continued: “Certainly once this crisis is over, I'll be looking for a lot of answers from a lot of people.”

CP apologizes to Calgarians

Following a conversation with CP’s CEO Hunter Harrison, Nenshi later said the company apologized for the chaos caused by the derailment.

"(Harrison) extended an apology to the citizens of Calgary for what has happened here," Nenshi told The Canadian Press. "We both agreed, No. 1, our primary responsibility is to get this thing cleaned up and, No. 2,  that we will work together much more and he reiterated safety in every community CP Rail runs through is a primary responsibility.

"I was happy to hear that commitment and now we'll see how well we're able to fix this problem."

The Transportation Safety Board of Canada said Thursday it will launch a full, independent investigation into the accident.

“On the ground we have two people out there at the moment looking at the cars, downloading the data recorder that’s on the locomotive, interviewing the crew and they’re going to be getting the inspection records,” Wendy Tadros, chair of the TSB, told Power Play. “Until we take a good hard look at those we won’t know what we’re dealing with.”

She continued: “I think it’s probably a pretty stressful situation out in Calgary, and this has added to the stress that they’re feeling in that city,” she added.

Following earlier concerns the bridge was sinking, acting fire chief Ken Uzeloc -- who visited the bridge early Thursday morning -- said the structure appears to have stopped dropping.

“We have engineers watching it right now (and) it has not moved in the last 45 minutes,” he said.

During a 90-minute visit to the scene earlier Thursday, Uzeloc said the bridge dropped nearly two feet.

Crews remain at the scene trying to determine the best way to get the remaining cars off the bridge.

“The first step we’re looking at is to secure the remaining railcars that are there, to ensure that if the bridge does completely collapse, the railcars will not be floating down the river,” Uzeloc said.

He said a parallel bridge will be inspected to make sure it’s in working order. A train will be brought in on that bridge to help pump out the liquid from the stranded railcars. Finally, a crane will be brought in to remove the railcars, once they’re emptied.

As an added precaution, cables will be used to secure the railcars to heavy machinery on land, so that the cars won’t drop into the river if the bridge gives out, he said.

Uzeloc said the situation is very dynamic, but if everything goes according to plan, the entire procedure could take between four to five hours.

Train officials realized something was wrong around 3:45 a.m. local time on Thursday.

Uzeloc said the majority of the train had made it over the bridge before engineers realized there was a problem.

A number of roads surrounding the bridge have been closed and a one-kilometre radius surrounding the bridge has been evacuated.

“The public is safe,” Uzeloc said, adding that due to the bridge’s location in an industrial area, only non-residential properties have been evacuated.

Railway companies are required to do their own safety audits and maintain their own safety records, which can be requested by Transport Canada.

Meanwhile, a water treatment plant in Bonnybrook was evacuated Thursday after wastewater began flowing directly through the plant.

Communities downstream have been alerted and the drinking water remains safe in Calgary, but Nenshi said “this is not a situation we like.”