At least four people, and possibly up to 55, were exposed to radiation when radioactive material leaked during a laboratory experiment in Japan, the facility's operators said, although authorities were not told of the leak for two days.
The radioactive material also leaked outside the nuclear physics laboratory in Tokaimura, 110 km (65 miles) northeast of Tokyo, during the experiment on Thursday, the Japan Atomic Energy Agency (JAEA) said in a statement.
The JAEA did not report the leak to the government's nuclear regulator until more than 34 hours later.
"We may have been too lax in our way of thinking and dealing with these nuclear materials," Taichi Miura, a director at the High Energy Accelerator Research Organization that runs the laboratory jointly with the JAEA, told a news conference.
The incident highlights how Japan is still struggling with nuclear safety issues after the 2011 Fukushima disaster and comes as Japan debates whether to re-start dozens of commercial nuclear reactors that have sat idle since the crisis.
Japan has imposed stricter safety measures since the meltdowns at the Fukushima Daiichi nuclear station north of Tokyo. Nuclear operators have only two of 50 reactors running while they await safety checks from Japan's new regulator.
The JAEA statement on Saturday said researchers were using a proton beam on gold to generate particles during the experiment.
All four of those confirmed to have been exposed were involved in the experiment but none of them needed to be taken to hospital, a JAEA spokesman said.
The radioactive material most likely leaked outside the facility because a ventilation fan was being used, officials said. A JAEA spokesman was not immediately able to confirm what types of radioactive substances were leaked.
Four men were exposed to radiation of between 0.6 and 1.6 millisieverts, the JAEA said. Another 51 people who were in the building at the time are being examined, it said.
Japanese law has set an annual radiation exposure safety threshold of 50 millisieverts for nuclear plant workers during normal operations.
Japanese officials were investigating the leak on Saturday.
Tokaimura is where Japan's commercial nuclear power industry began in the 1950s and is also the site of its worst nuclear accident before the Fukushima crisis in 2011.
In 1999, two people died after an accident at a Tokaimura uranium reprocessing facility.