There are a number of British references to their team pulling out, like this BBC article.

Sky doesn't say which American team is leaving and I never saw any articles about it, but last night, CNN did a piece on Fairfax County's team that I watched and they said they'd only be there one more day. Maybe even LA County Fire Rescue is also pulling out at the same time.

As I understand it, they are leaving not because they have given up all hope or because of radiation, but it's more a bureacratic thing about Japan not requesting their help beyond this point.