POTUS tweet seems to suggest that today's attackers were already known to the authorities.
This means the Police already know who is responsible and seemingly allowed them to go about their terrorist business.
Get them all off our streets today.
It's not rocket science.
You can take what Trump says and ignore it imo. How the heck would he know? He's been all too quick to blame Muslims for everything, and indeed to paint London as some kind of sharia state in the past. The man's a cretin.