Most of the 20-odd tons of the radioactive substance monazite that disappeared after being purchased by the former head of a trading firm was probably sold to a man in Toba, Mie Prefecture, investigative sources said Thursday.

According to police, Hiroshi Ikeda, 84, now head of a foundation under the jurisdiction of the Education Ministry, as well as others being questioned, are telling investigators that the monazite in question was sold to a former hotel operator in the city.

The Metropolitan Police Department dispatched officers to the Toba area to attempt to locate the buyer and track down the substance. Once the missing monazite is found, authorities said they would measure the radioactivity of the material as well as the area where it has been stored.

So far, police and the Science and Technology Agency have located around 17 tons of the roughly 40 tons Ikeda is believed to have imported 20 years ago.

Ikeda has told police he sold most of it to radium-spa operators in Japan.

Some 15 tons were stored at a house in the town of Tatsuno, Nagano Prefecture, while another ton or so was stored in a warehouse in Kumagaya, Saitama Prefecture. Another ton was discovered spread underneath the floor of the warehouse of a personal computer shop in Omiya.

Gamma ray tests have been conducted at all three sites, as well as at a warehouse in Ichikawa, Chiba Prefecture, where the Tatsuno monazite had been kept for roughly 15 years up until this April. Regulators said radiation levels were deemed harmless to humans.

However, the Omiya facility has been sealed off because gamma ray levels have reached 41 microsieverts, or up to 400 times the level naturally found in the environment. The owner of the warehouse, meanwhile, was examined at a local health center. Authorities were also studying the possibility that Ikeda violated laws regulating radioactive substances by not receiving authorization to move the monazite.

Meanwhile, the science agency set up a task force Wednesday to discuss ways to dispose of the monazite and of preventing a recurrence of the fiasco. Investigations have revealed that the agency was aware of the existence of the monazite last November but did nothing about it.

The issue came to light after powdered monazite was mailed in envelopes to 10 government offices, including the Prime Minister’s Official Residence, last week. Messages in the envelopes named an acquaintance of Ikeda who is allegedly selling uranium to North Korea.

Monazite is a phosphate that contains thorium, which can be used to produce nuclear fuel.