Arsonists destroyed six motorbikes worth £15,000 when they began a blaze at a garage.
During the fire at Russell Court, Ross Road, Wallington, on Friday June 12, a silver Norton with the index TSJ123 was also stolen.
Sutton detectives said the fire, which began about 3am, was started deliberately and destroyed the motorbikes which were put at about £15,000.
A police spokesman said the garage was in a block of garages used by residents of Russell Court.
He said a distinctive, older style Mercedes 550 coupe, red in colour, was seen in the area at the time and may or may not be connected with the incident.
The two-door vehicle had blacked out windows and a 'For Sale' sign in the rear window.
Detective Constable Karen Down, of Sutton CID, who is leading the investigation, urged anyone with information about the incident or the red Mercedes to get contact police.
Call Sutton CID on 020 8649 0747 or Crimestoppers anonymously on 0800 555 111.
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