though there is a crane there, they simply can't lift these slabs. it is like a puzzle. every piece that's pulled out -- >> and they're brittle. the idea of knowing that there are at least three people alive in there changed the dynamic entirely, because now you have to make sure that every little thing you move doesn't cause a collapse. >> and it's also a risk for the rescuers themselves. any one piece, which you can't engineer. you don't know how these large plates of cement -- the fact that these kids perhaps are protected by a concrete table or some sort of slab is unique. there are air bubbles and air pockets. yes, mariana? >> reporter: the barricade has gone back up. they're closing the perimeter once again, and it seems like that ambulance that was going to come out did not come out after all. we did see several more people, mexican federalis, go in.