Invincible Iron Man (2005 series) #6

Tony figures out Maya was Dr Killian's accomplice because in #4 it took 2 people (Maya and Tony) wielding 2 keys (Maya's and Killian's) to open the Extremis vault. So Killian couldn't have stolen the Extremis dose on his own. Because this 6-issue arc took nearly 1½ years to be completed, there are lots of Iron Man appearances to fit in before the next issue in this series. The full list can be seen in the relevant Official Index to the Marvel Universe (or online in the unofficial Marvel Chronology Project). I will single out, in sequence:- Illuminati #2, New Avengers #2-10, Young Avengers #7,8,11, issues of House of M including IM:House of M #1-3, NAv#11-15 and Annual #1, IM:Inevitable #1-6, and NAv#16-20.