Hearsay

It wasn't real evidence. There was no smoking gun. The old lady down the street said she heard what sounded like a bullet going off at midnight. But it couldn't be proven that the sound was linked with the gun shot that ended up splattering zombie brains in a surprisingly well organized and aesthetically pleasing mess on the white canvas that had been placed on the wall behind formerly moving Zombie Al.

The old lady's deposition describing said events was only hearsay and could not be used in court. Someone heard something. They said something. And, well, that was it. Especially for Al.