According to Rabbi Yaakov German, the amateur detective who found the video that led to confessed killer Levi Aron, Shomrim had a key video 8 or more hours before he did. But Shomrim wrongly decided the boy on the video was not Leiby Kletzky.
Two key points:
1. Shomrim checked surveillance video at 2 am Monday night Tuesday morning along the route Leiby took, and said Leiby wasn't on the video – but Shomrim was wrong. Leiby Kletzky was on that exact video, and Yaakov German discovered that 8 hours or more later. This mistake could easily have resulted in Leiby Kletzky's death.
2. Levi Aron going in and out of the dentist's office was only noticed after German and his friend Yehuda Borenstein watched the video repeatedly in slow motion, rather than in regular playback speed.
These two key points (less my comment about Shomrim's mistake) are found on the third part of the audio.
From last night's Talkline radio show with Zev Brenner. Please click the gray bars to listen: