The McCann's drugged Maddie to keep her asleep while they went out with friends, she choked on her own vomit and died, they knew if an autopsy was carried out the drugs would be detected and the other two children would of been taken by social services, so they got rid of the body.

Local authorities believed that the younger twins had been drugged as they barely moved while all the commotion was going on.

Also, the PI hired to investigate died under suspicious circumstances.