Complex sentence: If the source code is available, support can by provided most effectivly, sometimes the source code becomes available by intendation, sometimes by leaking or mistake like it happend for the Game engine of the Thief series.
Split into simpler sentences: If the source code is available, support can by provided most effectivly. Sometimes the source code becomes available by intendation, sometimes by leaking or mistake like it happend for the Game engine of the Thief series.