What I'm seeing pro-Israel sources saying is that the building is relatively intact and that the explosion took place in a parking lot. Given that Hamas stated "500 deaths" less than half an hour after the event, they also express some skepticism about how true the casualty count really is.
I'll only say that this is a total mess right now and that everything should be seen with some skepticism, especially claims that reinforce what you already believe or want to believe.