"(Dumbledore) was sure that somebody close to the Potters had been keeping You-Know-Who informed of their movements....Indeed, he had suspected for some time that someone on our side had turned traitor and was passing a lot of information to You-Know-Who."
-- Minerva McGonagall (PA10)
"DON'T LIE! ...YOU'D BEEN PASSING INFORMATION TO HIM FOR A YEAR BEFORE LILY AND JAMES DIED! YOU WERE HIS SPY!"
-- Sirius Black to Peter Pettigrew (PA19)
Dumbledore and his allies begin to realize that there is a spy in their midst but Peter is not suspected.