London Panic?

I can't tell if this is good policing or signs of panic, but I heard from at least one person that office workers in certain buildings in London are currently not being allowed to enter or exit.

I have no idea why the police would take this measure unless there were a chemical/biological threat (of which there are no signs or even speculation). I don't know how sealing people inside buildings would help with an investigation in any real way.

It seems to me more a sign of helplessness than anything else.

*Update*
Apparently this was a security precaution based on warnings around the St. Paul's area. Maybe today will be the straw that forces London to address its radical Islamic mosques...

Or maybe it will just be another reason to hate the U.S.