Something Going Down in the Mission Right Now

We just got an SMS that traffic (including Muni) is blocked on Potrero between 20th and 22nd, and that everyone is advised to avoid the area. What's going on? Anyone nearby? We can't find anything in the news about it.

UPDATE! Readers report lots of helicopters, and the evacuation of the hospital due to a suspicious device. What do you suppose the device is? A sandwich, or a speak-n-spell, or CAT scanner?