Baby blue tits attacked and killed in nest

When we switched on the nestbox camera yesterday a most gruesome sight was waiting for us! All seven healthy blue tit chicks had been ripped to pieces - literally torn limb from limb and body bits all over the inside. It was so upsetting I’ve disconnected the camera - don’t want my son to see it. It is a RSPB box with the blue tit size hole and we’ve had broods in there for the past four years without interference. What could possibly have done this? Husband thinks a mouse or rat but no idea how they could have got up that high into the tree. So worried it might happen again. What might have happened? The mum was attentive but no sign of dad for a few days so she had to leave them to find food.