Five years later, David Blair still doesn't know what the pilot was thinking.

Blair was part of a group of rowers on the Ottawa River in early August 2014 when a float plane landed just offshore from the nearby Ottawa Rowing Club.

The plane then took off near the rowers, flew beneath the Macdonald-Cartier bridge, and disappeared into the sky.

The incident prompted a Transport Canada investigation, but the agency was unable to identify either the plane or the pilot.

"We weren't sure if he was trying to kill himself," Blair recalled in a recent interview.

There were several boats on the water at the time, including some novice crews in large, hard-to-manoeuvre boats.

"If something did go wrong, they weren't going to be able to get out of the way," Blair said. "We were kind of peeved that the whole thing happened."

Peter Burpee sent in this cell phone video of a float plane taking off under the interprovincial bridge. 1:20

'Comprehensive investigation' turned up nothing

In eight years of rowing, Blair said he'd never seen a plane that far up the river. Rowers would occasionally spot one circling around the Rockcliffe Flying Club, about four kilometres away from the bridge, but not closer than that.

In a recent statement to CBC News, Transport Canada said it conducted a "comprehensive investigation" but came up empty.

"[The investigation] included reviewing video footage, interviewing witnesses and analyzing radar data. However, the department was unable to identify the float plane involved," the agency said.

To make a positive identification, the agency needs to know the aircraft's registration markings, the statement added.

At the time of the incident, instructors at the Ottawa Flying Club told CBC News that while taking off under a bridge isn't technically illegal, it is reckless.

Canadian aviation regulations say pilots are supposed to stay about 150 metres away from people and structures, except while taking off and landing.

If the pilot had been found to be flying recklessly and endangering anybody, including themselves, they could have been reprimanded, fined or had their licence suspended.