Apparently the fall/winter finale of The CW's Arrow will feature, in addition to Oliver Queen facing off against Ra's al Ghul and the League of Assassins, the true identity of Sara Lance's killer, finally revealed with a little help from The Flash's friends at S.T.A.R. Labs.
That's revealed by reading between the lines of a recent conversation that executive producer Marc Guggenheim had about the twist, in which he suggests that the S.T.A.R. team will help resolve the mystery by the end of the year...but not in during next week's The Flash/Arrow crossover event.
“I’ve always maintained that you’d find out the answer to who murdered Sara by the end of the year,” Guggenheim told Entertainment Weekly. “That said, I will say that the S.T.A.R. Labs team is going to end up providing a critical piece of information that cracks open the whole case, but that doesn’t necessarily happen in [the crossover].”
Since Arrow won't return until January, "the end of the year" is a pretty limited window -- and there's just the one episode after the crossover to get it all done in.
This weekend, it was revealed at the Arrow/The Flash crossover screening that the upcoming midseason premiere (set to air in January) will heavily feature Katie Cassidy's Laurel, in the first of a three-part storyline that will culminate with her taking on the mantle of Black Canary. It's safe to assume that the long-rumored story of how Sara came to be Canary following her apparent death on Lian Yu will be told in flashback during one of those three episodes.