London transit authorities say they are examining a video that appears to show a tram driver asleep at the wheel on the same route where a tram with a different driver derailed this month, killing seven people.

The Sun newspaper on Saturday published the footage, which it says was taken by a passenger in April.

London Mayor Sadiq Khan says he is very concerned by the video. Transit authority Transport for London and tram operator First Group say they are urgently investigating.

Rail accident investigators say the derailed tram was traveling at 44 mph (70 kph) in a 12 mph (20 kph) zone when it came off the tracks Nov. 9.

One theory being investigated is that the driver lost consciousness.

The driver was arrested on suspicion of manslaughter.