Weren't they planning on herding everyone to the roof and blowing it up?
The plan was to do that to cover their getaway. Herd the hostages up to the roof, nominally to stop snipers while they got in a helicopter. Then flow it all up. By the time they sorted the body parts and realized they weren't there, they would be gone and safe by a different route.
This is also why getting the detonators back was so important. Without them, they couldn't blow the roof.