Telluriuim-132 was detected 6 kilometers northwest of Fukushima I Nuke Plant in Namie-machi in the morning of March 12.
That's before the Reactor 1's reactor building blew up (March 12 late afternoon), and even before TEPCO managed to do the venting (March 12 early afternoon) for the Reactor 1.
Nuclear and Industrial Safety Agency, regulator for the nuclear industry, decided to sit on the data for 2 months and 3 weeks. Agency spokesman Nishiyama's excuse? "It never occurred to us to disclose the data."
The Japanese government is doing "one heck of a job" indeed, as IAEA's over-the-top praise indicates (from IAEA's preliminary report):
"The Japanese government's longer term response to protect the public, including evacuation, has been impressive and extremely well organized."
Yomiuri Shinbun (11:09PM JST 6/3/2011; emphasis is mine):
It was disclosed that the radionuclide that would indicate the nuclear fuel temperature exceeded 1,000 degrees Celsius was detected in the morning of March 12 in Namie-machi in Fukushima Prefecture, about 6 kilometers [north] from Fukushima I Nuclear Power Plant.
The Nuclear and Industrial Safety Agency (NISA) of the Ministry of Economy, Trade and Industry (METI) announced on June 3. Asked about the disclosure after more than 2 months after the accident started, Agency spokesman Hidehiko Nishiyama defended his agency by saying "We didn't intend to hide the information, but it never occurred to us to disclose it to the public. We are sorry." The government's commission on the Fukushima I accident may investigate the incident further.
The radionuclide detected was "tellurium-132", and it was found in the dust particles in the air. It was detected before the venting of the Reactor 1, which released radioactive steam.
By the way, University of California Berkeley was detecting tellurium-132 in the air from March 18 to March 29. Is it any wonder that it was detected at a location 6 kilometers from the plant, when it was detected across the Pacific Ocean?
Tellurium-132's half-life is 3.2 days.
That seems to me to indicate that fission was ongoing long after the SCRAM (March 11). I'm still patiently waiting for NISA and the government to admit recriticality had occurred.