Why do the authorities feel it necessary to seal off such a large area when
they have already made the bomb safe and removed the explosive material?

Someone said that it was so the police could forensically search the area
for stuff the suspect might have dropped. However there have been no teams
of police on hands and knees conducting fingertip searches or anything like
that.

Or is it not really needed at all and its just to make a statement to the
public about how serious it could have been?