Multiple bomb threats were received at Canadian airports Thursday morning, prompting flight delays, evacuations and investigations into the source, according to NAV Canada and local authorities.

“Early this morning, NAV Canada was informed of bomb threats affecting facilities in Ottawa, Montreal, Edmonton, Winnipeg, Calgary, and Vancouver. Employees at the affected locations are safe, and a temporary ground stop is in place at the affected airports,” NAV Canada said in a statement.

NAV Canada provides air traffic control operations to Canadian airports.

“We are working with authorities to address the situation and ensure the safety of all staff and passengers. Travelers are advised to check directly with their airline for the latest updates as delays may occur,” the statement added.

A spokesperson for Montreal-Trudeau International Airport told Global News that some flights were affected after the NAV Canada control tower was evacuated, but service has now resumed.

Toronto Pearson, Canada’s largest airport and one of the most important in North America, has not been affected by the threats, which are being investigated by law enforcement.

Meanwhile, Reuters reported on investigations into security incidents at the Ottawa International Airport and Montreal-Pierre Elliott Trudeau Airport, which could disrupt operations.