"When did Snape turn?" Ginny asked.
Sirius looked out the window, as if trying to decide how much to say. "We don't know for sure, but I have suspicions. You've heard the prophecy, right?"
"Yeah?" Harry said.
"Well, someone else heard it too, at least a part of it, and that's what prompted Voldemort to attack you. We know Pettigrew told Voldemort that you were a Sport, but someone else leaked the prophecy. Someone who was physically present when Trelawney made the prophecy."
Harry and Ginny both made the connection at the same time. "Snape?"
"He was caught listening to Trelawney and Dumbledore. He went to Voldemort, and then discovered that the prophecy meant the only person he ever liked was targeted. We don't know for sure, but we suspect Snape then went to Dumbledore to try and save her. At least, that's what Zeus believes, and I agree with him. He's supposedly been Dumbledore's man ever since."
"The timing doesn't make sense, though," Ginny said. "Last year, Luna showed us Wizengamot transcripts that said Snape turned spy and did a lot to serve the light. But…but he didn't turn to Dumbledore until right before the end of the war. What did he actually do? He got Harry's parents killed and then felt bad about it, but what did he do to make up for murdering his father and betraying his only friend?"
"That," Sirius said, "is the big question. And the answer is that no one knows."