U.S. authorities are trying to clear up what happened after a spilled cup of coffee led to an emergency landing at Canada's largest and busiest airport this week.

The United Airlines flight from Chicago to Frankfurt, Germany, was diverted to Toronto on Monday night when the plane began to experience problems with its communications equipment.

The problems arose after the pilot in command spilled coffee on the electronic controls in the cockpit's centre console.

When an emergency hijacking alert was accidentally sent out from the cockpit, the crew decided to cut the flight short and land the Boeing 777 at Toronto's Lester B. Pearson International Airport to have the communications problems fixed.

The plane landed without incident or injury to any of its 241 passengers.

As a result, the U.S. Federal Aviation Administration is trying to confirm the chain of events and is looking into United Airlines' policies to find out if the flight crew breached any procedures.

FAA investigators will confirm whether the false distress alarm was caused by the spilled coffee or something such as human error.

FAA spokesman Les Dorr said investigators will also probe whether United Airlines has a policy that prohibits drinks from being held over the centre console, and whether the airline explained such a policy to flight crews.

Dorr said food and drinks are allowed in cockpits but meals are prohibited during critical moments, including a plane's takeoff and landing, to avoid distractions.

It is open to debate whether drinking coffee is considered eating a meal, but it's not unreasonable to suggest coffee might be considered a distraction, he said.

"Pilots must devote full time and attention," Dorr told CTV.ca by telephone from the FAA's headquarters in Washington, D.C.

A spokesperson for Chicago-based United Airlines wasn't immediately available for comment.

As for the passengers who made the unexpected stop in Toronto, they boarded a different plane and were taken back to Chicago, where they boarded another flight to Frankfurt on Tuesday.