Security officials say a screening machine software malfunction caused the two-hour shutdown that stalled thousands of travellers at Vancouver International Airport last Sunday.

However, a spokeswoman says they're still not sure how it happened.

Dozens of flights were grounded for more than two hours after an X-ray image of a bag, with an explosive inside, showed up the monitor of a screening device.

A security officer then triggered an alert that went to Transport Canada, the RCMP and the Canadian Air Transport Security Authority (CATSA).

All the passengers were rescreened, and 50 planes —  including several that had already taken off and were called back — were grounded for more than two hours. Incoming flights could land during the clampdown, but people were not allowed off the planes.

Bag didn't exist

The bag with the explosive was never found because it didn't exist. CATSA's Renee Fairweather said the image that showed up was from training software installed on the screening device.

She said it should never have appeared, except during a training exercise.

Fairweather said the agency has disabled the software on screening machines at airports across the country to prevent a recurrence.

"They're investigating how that feature of the tool got inadvertently activated. And while they're doing that investigation, they've deactivated the tool itself. "

Fairweather said the change won't affect the normal operation of the screening machines.