What's Politico's source for claiming 10,000 protesters gathered in D.C. today?

I'm curious because Politico states the number as fact without providing additional information:

The crowd grew to about 10,000 by noon – a significant gathering for a weekday but far less than the 9/12 protests earlier this fall.

Politico's Jonathan Martin clearly seemed impressed that 10,000 people showed up to protest health care reform. But did 10,000 people actually show up? And more importantly, where did that number come from? As we've seen in the recent past, crowd estimates for political rallies can take on a greater significance, and therefore journalists need to be as precise as possible when reporting on the figures. Instead, Politico simply announced 10,000 protesters had rallied without explaining where that figure came from.

By contrast, here's how MSNBC's First Read reported on the rally:

NBC's Luke Russert, reporting from the West Front of the Capitol, passes along this photo of a cartoon Pelosi with the words "UNAMERICAN MCCARTHYITE" scrawled across. The crowd, per Russert, is so far about 3,000 to 3,500, according to Capitol Police estimates...

*** UPDATE *** Three Capitol Hill police officers all guessed that the crowd numbered at about 4,000.

NBC, relying on estimates from local police, put the crowd number at 4,000. So what source did Politico rely on when it arrived at the much larger 10,000 figure?

Curious minds want to know.

UPDATED: Gateway Pundit, last seen lying about the size of the Sept. 12 rally, claims today's protest was "MASSIVE."

Really? 4,000 people, the size of a crowd at a minor league baseball game, now qualifies as "MASSIVE"?