What Was the Deal With the Helicopters Yesterday?

A reader writes:

Does anybody know what these large helicopters were for? Spotted around 4:30 pm on 4/4, flew in from the north, landed in McCormick parking lot for about 10 minutes, and then flew back north. The 3 large ones said United States Marine Corps.

Not sure what this is for, but in the past President Obama took Marine One to Northerly Island to avoid traffic (and avoid shutting down the highways).

(Hat tip:  JW!)