REVEALED: St. Louis school shooter, 19, was a former student who returned to gun down teenager, 16, and her female teacher - while police REFUSE to say how he got into 'hardened' building because it would 'give a blueprint' to future killers

It's not exactly rocket science how he got into the school. Maybe I don't have enough imagination, but I can only see 3 ways that he got in.

1) Someone let him in.

2) The doors didn't lock for some reason. Was it a maintenance issue or were they left unlocked due to incompetence, apathy, etc.?

3) The shooter came to school normally, left through a normally locked door, and blocked the door from closing as he left (to return with a gun).