Can I activate found iPhone?

Here's the story:
Last summer, I found and iPhone 4 when I was at a park with some friends. I posted ads on Craigslist and never got a legitimate reply. I also posted fliers on local business/grocery store bulletin boards and also got fakers trying to gain a free iPhone. It didn't have service when I found it. It did obviously belong to someone though, it had a photo customized case on it. Could I activate it without being accused of theft?

No, because all iphones have serial numbers which are registered with service providers. My service provider has the serial number to my phone and if someone tries to activate it, they will see that it has been reported stolen. If you bring it somewhere else the same thing could happen if the owner calls all service providers and gives them the serial number.

Find the owner.