He failed a field breathalyzer test and was then arrested and given another test, said Michael Conway, Director of Public Affairs at the Detroit Metropolitan Airport. The blood alcohol content was not released, but Conway said it was over the limit for a pilot.
FAA rules state that pilots cannot fly with a blood alcohol content of 0.04 percent or greater.
American Airlines said in a statement: "This is a serious matter and we are assisting local law enforcement and the Federal Aviation Administration with the investigation. We will handle this matter appropriately as the safety and care of our customers and employees is our highest priority."
The airline said Flight 736 was cancelled and customers were being reaccommodated.
"We apologize to our customers for the disruption to their travel plans," the airline said.
It's unclear where the co-pilot was at time of arrest, Conway said, but the airport police were called at 6:40 a.m. and the plane was due to take off around 7 a.m.
The co-pilot's identity was not released but Conway said he was born in 1965 and is from Pennsylvania.