The unnamed worker died in early August, after having worked at Fukushima I for about a week. TEPCO's Matsumoto says the company does not know where the worker had worked before he came to Fukushima I. He's from one of TEPCO's 1st-tier subcontractors.
Since the death has nothing to do with having worked in Fukushima I Nuke Plant, TEPCO is not going to track the worker's past work history or conduct further investigation. TEPCO will not release personal information about the worker, either.
The radiation exposure of the worker was 0.5 millisievert external radiation, and zero internal radiation for the duration of his work at Fukushima I.
Additional information from Mainichi (8/30/2011):
The worker was in his 40s. At Fukushima I, his work included manning the rest area and radiation control of the workers. There was no abnormal reading of white blood cells before he started to work at Fukushima I. He worked for about a week in August, and fell ill. He was diagnosed with acute leukemia, and died at the hospital. His company reported his death to TEPCO on August 16.
TEPCO's Matsumoto emphasized that the death was a private matter, and TEPCO had no intention of investigating it further now that the doctor denied any relationship between the death and the work at Fukushima I.