Janna killed Yasuo's master, not Riven

Ok so we all know Yasuo was framed for killing his master during the war between Noxus and Zaun against Ionia, but was it really Riven? Sure both share a wind style, which matches the torn up master, but Riven quit mid fight to flee because the Zaun front was killing her people in the process, and it seems unlikely she was the killer.

However I think the killer was someone else. Before Janna went to Piltover, she was an orphan from Zaun. How do we know whether or not she was used during this war thus potentially being the killer?