ten second count starts when?

[Nevadaref]

Firedoc and JRut are certainly correct that the ten-second count can only start after team control is established. However, from the original question it seems that Team A does have team control since A1 is passing to A2. The deflection by team B does not end this team control, so, on this play, the ten-second count should start as soon as the ball gains backcourt status.
Now if the pass from A1 to A2 is a throw-in pass, then you wait until a player inbounds establishes control before there could be a ten-second count. But the original question doesn't say if this pass is a throw-in pass.