The Rail Accident Investigation Branch (RAIB) said the tram was travelling at a “significantly higher speed than is permitted”.
The train, travelling from New Addington to Wimbledon, derailed at a sharp curve with a 20 kph speed limit as it was approaching a junction.
Deputy Chief Constable Adrian Hanstock said: “This is a tragic incident and our hearts and thoughts go out to all those affected. When officers arrived on scene this morning, they were met with a complex and challenging situation.”
British Transport Police officers are continuing to work at the scene of a major incident in Croydon.
Wednesday’s accident was the greatest loss of life in a British rail accident since seven people were killed and 76 were injured when a high-speed train from London came off the tracks at Potters Bar station in 2002.