A short article by Kyodo News doesn't reveal much details, but it is clear, sort of, that the US was measuring radiation from the Fukushima I Nuclear Power Plant accident from very early on.
Kyodo News writes as if everybody knew there was a team of US experts measuring radiation around Fukushima I Nuke Plant right after the accident. (Did we?)
Kyodo News (12/31/2012):
US sent a nuclear special team to Japan, first [overseas] deployment of the team; Kan administration did not know about the deployment
It was revealed on December 31 that a team that the US government sent to Japan in order to measure radiation around Fukushima I Nuclear Power Plant right after the start of the nuclear accident was a special team to counter nuclear terrorism. It was the first overseas deployment for this team, but the core officials in the Kan administration weren't aware of the deployment at that time. The actual measurement [as opposed to simulations like SPEEDI] by the team was therefore delayed, and it may have caused "unnecessary radiation exposure" for the residents (Namie Town Assembly chairman).
Multiple sources in the US government and the Japanese counterparts who were involved in the deployment disclosed the news to Kyodo Tsushin.
The team consisted of 33 people in "Damage Control and Response Team" [my translation, probably not the official name] in charge of measuring gamma radiation from the sky and analyzing the situation of contamination.
Could this team be from Nuclear Emergency Support Team? Kyodo article has a photograph provided by the US National Nuclear Security Administration, so it is likely it was this team.