She may have accidentally broken its neck or back. This can happen if the animal is climbing around in the dresser and the child pulls open the drawer. She probably isn't telling you because it was traumatic. Just comfort her. She will tell you some time soon.
I think chances are high that she was trying to be a little mean, and it died. I think if it were a pure accident, she wasn't looking and stepped on it, or she was lovingly carrying it and it jumped out of her hands, she would have come screaming to you for help.
I will say, this happens a LOT with little kids and it doesn't mean they're monsters. The fact that she was crying, and felt remorseful, and was too ashamed to tell you what she'd done means she has a tender heart, just a lapse in judgement.
When I was her age I had this parakeet. I had him out to play with him and tame him, and when it came time to put him back in the cage he wouldn't let me catch him (he didn't have clipped wings). So anyway, I was trying to get it back in the cage and realized I'd have to not allow it to rest until it was too tired to fly away and I could catch it - so I had this towel and I was "snapping" the towel behind the bird so it would fly off the top of curtain rods, and the top of my dresser mirror. I never intended to hit him. Horror of horrors, when I snapped the towel it actually HIT the bird and killed him. I told my mom the bird had flown into the dresser mirror and died. I still feel bad about that but honestly, I never meant to hurt it.
I think your daughter was doing something kind of like that. If you never see any more signs that she is mean to animals, I think you should let it go.