I had to put my dog in the pound? and he's not there anymore?

Well he's a a year old pitbull/mastiff mix and we had to put him in the pound because he bit my sister and i was going back to get him because my friend was going to get him but he wasn't at the pound anymore? did the pound put him to sleep or something? what do you guys think or do you think some1 else took him? asked the vet already and they can't say anything for some reason what do you guys think? isn't it a certain time though...before they can put the dog down? i feel soooo sad