Police in the US state of Michigan aren't ruling out laying further charges, over America's deadliest school shooting this year

15-year-old Ethan Crumbley has pleaded not guilty to a slew of charges over the shooting in suburb Detroit -- in which four students died and another seven people were injured.

His parents have also pleaded not guilty to charges of involuntary manslaughter, and have denied giving the boy free access to the gun.

Police say the pair were found hiding in a warehouse after cashing out their bank account, and the warehouse owner could also be facing charges.