A worker at Japan’s Fukushima Daiichi nuclear plant died Saturday, the company operating the plant said, marking the first casualty during the ongoing effort to stabilize the tsunami-battered facility.

Although the cause of the death remains unknown, the worker’s body showed no sign of radiation overexposure, according to Naoyuki Matsumoto, a spokesman for Tokyo Electric Power Co.

Still, the death renews concerns about conditions at the plant — a recent target of criticism by government officials and doctors who have checked employees. Engineers and technicians often must work in cramped, sweltering areas, rotating out quickly to limit their exposure to radiation.

Tepco said the man, a subcontractor in his 60s, had begun work at the plant only a day earlier. He was exposed to 0.51 millisieverts of radiation Friday and an additional 0.17 millisieverts Saturday — in all, roughly four times the dose from a chest X-ray — and no radioactive material was found on his body. He was wearing protective gear, including a mask, when he collapsed Saturday morning after about 50 minutes of work. By the time he was taken to a doctor’s room at the plant, he was no longer breathing. The cause of the death is being investigated by police.

According to Tepco, the man had said earlier in the morning that he did not feel well. At the time of his collapse, he was working in a disposal building where radioactive water is stored.

“The temperature in the facility is not being monitored, but we hear that it is not a severe working environment in terms of humidity and radiation levels,” said Yoshimi Hitosugi, another Tepco spokesman. “We also understand that the work itself did not impose a high physical burden.”

As of Friday, there were about 1,900 people working at the Daiichi plant.

Amid the worst nuclear emergency in a quarter-century, Tepco has already dealt with several worker health problems — though they pale in comparison to those at Chernobyl, where several dozen workers died within weeks of the accident because of acute radiation exposure.

At Fukushima, two workers were hospitalized in late March with radiation burns after stepping into contaminated water. They were not wearing boots at the time.

Earlier, two workers at the facility died during the initial double disaster — a 9.0-magnitude earthquake followed by a tsunami — that crippled the plant March 11. The employees, who were working in a power panel room when the tsunami hit, were not found until March 30.

Five days after the disaster, Japan’s Ministry of Health, Labor and Welfare raised the allowable radiation exposure levels for nuclear plant workers, from 100 millisieverts per year during emergency situations to 250 millisieverts per year.

Saturday’s death occurred as Tepco continued its efforts to replace the plant’s cooling system, which was knocked out by the tsunami. The utility last month revealed a road map for stabilizing the plant in six to nine months. But government officials are now saying that could be overly optimistic, particularly in light of new data that show worse-than-expected damage at the No. 1 reactor, where fuel rods were left fully exposed.

Special correspondent Akiko Yamamoto contributed to this report.