Tokyo (CNN) -- An army of Japanese police investigators on Wednesday raided the uranium processing plant where the nation's worst nuclear accident occurred last week, as a criminal investigation into the cause of the radiation link widened.

Police entered the plant at Tokaimura, 140 km (90 miles) northeast of the capital, to investigate alleged professional negligence and violation of nuclear safety laws. Investigators also searched the plant operator's Tokyo headquarters and interviewed dozens of workers and management officials.

The September 30 accident contaminated 49 people and forced 310,000 living near the plant to stay indoors for two days to shield themselves from possible fallout.

Meanwhile, Japan's Kyodo news agency reported Wednesday that the government plans to revoke the business license of the plant operator. The plant is operated by Tokyo-based JCO Co., a subsidiary of Sumitomo Metal Mining co.

"The agency decided to revoke JCO's license due to the seriousness of the accident," Kyodo said, quoting unidentified sources at Japan's Science and Technology Agency.

Ibaraki Prefecture Police limited their search at Toikaimura on Wednesday to an office building and other locations. They stayed away from the nuclear fuel building because of concerns about its safety.

The accident began there after workers reportedly put a large amount of a powdered uranium compound in liquid nitric acid in stainless steel buckets, instead of using the proper melting equipment for creating a chemical reaction.