Complex sentence: The brothers collected stones and bricks which were thrown at the boys' heads; when alerted by the sound of passers-by, the suspects covered the two boys with a sheet which they then set alight, causing burns to the victims.
Split into simpler sentences: The brothers collected stones and bricks which were thrown at the boys' heads. When alerted by the sound of passers-by, the suspects covered the two boys with a sheet which they then set alight, inflicting burns on the victims.