An area of grassland in Sheffield was set alight three times in 14 hours.
Crews were called to Edge Well Drive, Birley Edge, at 8pm on Sunday, at 9.45pm that night and at 5.20am today.
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Firefighters also dealt with blazes at 8am on Saturday and 9am on Sunday but both incidents were logged as accidental.
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Anyone with information on the arson attacks should call South Yorkshire Police on 101 or Crimestoppers on 0800 555111.