OTTAWA—A drone? A weather balloon? A piece of garbage swept up by the wind?

The pilots of a Porter Airlines flight inbound to Toronto on Monday morning were forced to put their plane into a sudden dive to avoid a mid-air collision with an unidentified object high over Lake Ontario, causing minor injuries to two flight attendants.

Now investigators with the Transportation Safety Board of Canada admit they have a puzzle on their hands to determine exactly what the pilots had to avoid.

“Nobody knows at this point. It happened so quick,” said Peter Rowntree, senior regional investigator with the safety board.

“We’ve got our work cut out trying to figure out what this unidentified flying object was. What did they encounter?” Rowntree said in an interview.

“It definitely wasn’t a bird. It was a fairly large object,” he said, describing it as up to three metres wide.

There were initial concerns that the object was an unmanned aerial vehicle (UAV), or drone, highlighting once again worries about their growing prevalence and the risks they pose to aviation.

Rowntree said it was too soon to say for sure. But he said the location of the incident — at 2,743 metres (9,000 feet) over the lake near the U.S. border, far from shore — is reason for concern if it was a drone.

The Porter Dash 8-400 turboprop was on its initial approach to Toronto’s Billy Bishop Toronto City Airport from Ottawa when the two pilots saw they were on a collision course. They had mere seconds to spot the object and react to avoid the danger.

“They essentially just pushed the nose down to get underneath it,” Rowntree said.

“It’s a big sky out there, so the fact they even saw it is something,” he said, noting that the aircraft was travelling at almost 450 kilometres per hour.

Two flight attendants suffered minor injuries in the evasive manoeuvre. They were taken to hospital and released. None of the 54 passengers on board the flight were injured, airline spokesperson Brad Cicero said.

At first the pilots believed the object was a balloon, but later thought it could have been a drone, Cicero said.

The proliferation and capability of unmanned aerial vehicles has been a growing worry for pilots who fear a catastrophic mid-air collision. By Sept. 1, there had been 82 reports of potential UAV infractions in Canada this year, compared to 97 in all of 2015 and 61 in 2014, according to Transport Canada.

Transport Minister Marc Garneau’s office said Monday that “‎new, clearer regulations” for drone use would be released in the coming months.

Transport Canada recently launched an awareness campaign to encourage drone operators to fly their vehicles safely — and away from flight paths and airports.

Existing rules already restrict the use of drones around airports and at higher altitudes to minimize the risk of a potential collision. But there are calls for Transport Canada to do more to raise public awareness.

“I don’t believe the word is getting out,” said Dan Adamus, Canada board president of the Air Line Pilots Association, International.

“That’s the problem. People just look up in the sky and they don’t understand it’s airspace that aircraft use,” he said in an interview.

He said a mid-air collision with a drone would likely be worse than hitting a bird and could easily disable a control surface, cause an engine failure or shatter the windscreen.

And he said the pilots association wants Canada to follow the lead of the United States and require owners to register their UAVs with the federal authorities, a formality that may encourage some to be more responsible in how they fly the vehicles.

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“We certainly recognize the potential of remotely piloted aircraft for a whole variety of reasons, but they can’t compromise aviation safety,” he said.

Rowntree echoed that concern.

“It’s only a matter of time before — and you hate to say it — an airplane is going to hit a drone,” he said.