A British Airways flight that left Las Vegas and was headed for London made an emergency landing in Montreal because of a bomb threat early this morning.

The flight left McCarran International Airport at 9:30 p.m. PT Tuesday. It was due to arrive at Heathrow Airport at 3:25 p.m. BST, but several hours into the flight, the captain announced to the passengers that they were being diverted because he received notice an "unknown device" was on the plane.

Passenger Timo Berger was asleep when the announcement was made. He said the captain didn't say the word "bomb," but flight attendants did.

Berger said that shortly after the announcement, the plane descended to 6,000 metres.

"[They] told us if something happened, we would have breathable air and the effect of an explosion would not be so hard," he said.

"Some people were scared and afraid. Some people cried. But there was no panic on the aircraft," he said.

Canine unit helps with search

Flight attendants searched the plane while it was mid-air, hoping to find a suspicious device, but none was found. The plane landed safely in Montreal around 6 a.m. ET, and all 312 passengers disembarked safely.

Montreal police's SWAT team and canine unit spent the morning searching the plane, passengers and baggage.

A spokesperson for Montreal's airport authority said no bomb was found during the search.

Montreal police cars on tarmac, exclusive <a href="https://twitter.com/hashtag/BA274?src=hash">#BA274</a> pictures sent to me by passenger <a href="https://twitter.com/hashtag/cbcmtl?src=hash">#cbcmtl</a> <a href="https://twitter.com/CBCMontreal">@CBCMontreal</a> <a href="http://t.co/mawtXzPJyV">pic.twitter.com/mawtXzPJyV</a> —@RebeccaUgolini

Police said the evacuation went smoothly and confirmed they received a phone call about 40 minutes before the plane landed.

A spokeswoman for British Airways said the captain made the decision to divert the airplane as a precaution.

The airline said it is co-operating with Canadian authorities.

The passengers were taken by shuttle buses to nearby hotels. Most of them will be booked on a flight to London on Thursday.