MONTREAL — A passenger who slipped past security with a knife in his bag was finally apprehended Saturday by security at Montreal's Pierre Elliott Trudeau International Airport.

But the security breach delayed international flights at the airport for over three hours on Saturday night, as authorities searched airplanes one by one for the traveller.

Montreal police said about 20 planes and thousands of passengers were affected by the investigation. Initially, security personnel didn't know if they were looking for a man or a woman.

Lynne Labelle of Montreal police said the tool, a set of multifunctional pliers that contain a knife, was noticed only after the man left the security area to catch his flight.

"He didn't know he didn't have a right to have this,'' Labelle said of the pliers.

The tool was spotted in the man's backpack after it passed through an X-ray scanner, but he was already gone.

"There was a delay (after) . . .  they saw the knife and so the man had the chance to leave,'' she said.

She didn't know what caused the delay at the checkpoint.

Labelle said it's unlikely the man will face charges.

Flights resumed their regular schedule after 9 p.m.

Meanwhile, a spokeswoman for the Crown corporation responsible for passenger screening at airports said Transport Canada will investigate the incident.

"This is another case where passengers who are not aware can cause huge, huge disruptions like we saw tonight,'' said Anna-Karina Tabunar of the Ottawa-based Canadian Air Transport Security Authority, which trains airport security personnel.

"The lesson to be learned out of this, and the message to other passengers is, to really be aware of what you can and cannot bring onboard with you, because even something as innocuous and innocent as a Swiss Army Knife, which is a prohibited item, can create enormous delays and headaches.''

She said even though there was no threat to passengers, security situations are taken seriously.

"If there's any doubt at all, then measures like this have to be taken,'' she said.

August is the busiest time for air travel in Canada, Tabunar said. Last year, more than 10 million air travellers hit the skies in August.