Inbounder possession

[TriggerMN]

B1 scores a basket, and the ball rolls out near the free throw line. A1 picks it up, and throws the ball to A2, who is standing OOB to catch the ball, and then throw it in. However, the ball bounces off A2's hands or chest or whatever (no possession) to B1 in the court of play, and B1 scores another basket.

Do you blow the whistle, wave off the basket, and give team A a throw-in? Must the inbounder have possession? Or is B1's second basket good?