The Washington Post story was double-bylined (and also credited several other reporters and researchers), and the lead byline went to Barton Gellman, who used to be full-time at the WP but (AFAIK) is at TIME magazine. Not sure what the publishing process for this story was about (it seems unusual to have a story lead-bylined by only non-staff members), but I imagine it had something to do with the Guardian dropping their bombshells and the WP deciding that it also should publish (perhaps this story had been in the works, but wasn't originally going to be published this week).

disclosure: I know Gellman as an acquaintance through a colleague and consider him to be a great reporter...but I'd probably say that anyway just based on him wining the Pulitzer for his reporting on Dick Cheney (http://www.pulitzer.org/citation/2008-National-Reporting)