What's the WashPost's source for claiming 10,000 protesters gathered in D.C. on Thursday?

Like Politico did yesterday, WashPost states as fact that 10,000 health reformer haters gathered in D.C. And like Politico, the WashPost provides no sources for that (generous?) estimate:

With the stage set for a historic House vote on health-care reform this weekend, an estimated 10,000 conservative activists descended on Capitol Hill on Thursday for a campaign-style rally in a last-ditch effort to defeat a bill they demonized as "Pelosi-Care."  

Question: Rather than relying on official D.C. estimates, did the WashPost simply rely on Politico, which seemed to rely on nobody for its 10,000 number? i.e. Was the WashPost just cribbing off Politico?

As we noted yesterday, here's the proper way to report on political protests, courtesy of MSNBC's First Read [emphasis added]:

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.

Isn't that fascinating? NBC relied on actual police estimates and reported the crowd was 4,000 strong. Politico and WashPost relied on phantom sources and reported the crowd was 10,000 strong. Which report seems more credible?

UPDATED: Do we even have to mention that right-wing media outlets invented all sorts of numbers for Thursday's modest protest? (Two million? Do I hear 2 million??) The WashTimes, for instance, went with "tens of thousands." But we expect that from propaganda outlets. The WashPost and Politico though, ought to be much more precise in their reporting.