As the investigation continues into last week's nuclear accident in Japan, the government and environmental activists are increasingly concerned that it may have been more serious and affected more people than initially reported.

The government has decided to expand its examination of people who may have been exposed to radiation near the uranium processing plant in Tokaimura, about 75 miles northeast of Tokyo, according to a spokesman for the Science and Technology Agency. To date, 63 people have been identified as having been exposed, including 14 workers who entered the plant briefly in an attempt to halt the nuclear reaction, and the three involved in the accident. Two employees remain hospitalized in serious condition.

"Initially we did not see the accident as being so serious," said Masaru Hashimoto, governor of Ibaraki prefecture, where it occurred. The incident has been characterized as Japan's worst nuclear mishap.

Officials also said they are likely to raise the accident's rating from Level 4 to 5 on the international scale of 7, the same as the Three Mile Island nuclear accident in Pennsylvania in 1979. Such a move would indicate that they believe the risk of contamination outside the plant was extensive.

The environmental group Greenpeace said today that, based on its analysis of samples taken 500 yards from the plant, beyond the area the government evacuated, the number of people exposed to radiation was higher than government estimates.

Jan Rispens, an energy specialist with Greenpeace, said the government should be testing people more thoroughly. "It's not enough to run Geiger counters over their arms and their feet," he said.

Details of extensive safety violations have come out daily since the Sept. 30 accident. Officials of JCO Co., which operates the plant, have told reporters and admitted to investigators that workers have been using the same illegal shortcut that caused the accident for the past seven or eight years. Company officials also said they had not made any preparations for this type of accident--in which an excess of uranium poured into a mixing container triggered a nuclear chain reaction.

Rispens said the plant "had the safety standards of a bakery and not a nuclear facility. It was just a normal building."

The reaction apparently continued for 17 to 20 hours, until workers succeeded in emptying water from the tanks and pouring in boric acid. Families living near the plant were evacuated, and more than 300,000 other people were asked to stay indoors for more than 24 hours.

On Wednesday, police searched JCO's headquarters in Tokyo and its office in Tokaimura and carted off boxes of documents. The search warrants cited suspicion of professional negligence, a police spokesman said.

Japan relies on nuclear power for about a third of its energy needs, but the nuclear industry has been plagued by a series of accidents.

Prime Minister Keizo Obuchi has reaffirmed recently the government's determination to push ahead with its nuclear power program. To demonstrate his faith in its safety, he visited Tokaimura and ate the local produce. But he also said he would seek a new law to spell out a response to nuclear disasters.

Special correspondent Shigehiko Togo contributed to this report.