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Envelopes containing small amounts of a radioactive substance were mailed last week to Prime Minister Yoshiro Mori’s Official Residence, as well as to the Science and Technology Agency and seven other government offices, police sources said Monday.

The Metropolitan Police Department is investigating the cases, in which envelopes containing radioactive powder — in quantities believed not harmful to humans — were delivered sometime between June 6 and 8.

An initial examination indicated the substance was ground monazite, a mineral containing thorium, the sources said. Each envelope contained several grams of thorium, which is a nuclear fuel material.

Chief Cabinet Secretary Mikio Aoki confirmed that the mail delivered to the Prime Minister’s Official Residence on Thursday contained about 3 grams of the radioactive substance in ground form. “Its radioactivity was estimated to be one microsievert and there will be no direct impact on human health,” he said.

The envelopes, postmarked in Tokyo but without the sender’s name or address, also contained a message written in Japanese warning that the contents were radioactive, the sources said.

In the message, the sender also reportedly alleged that uranium is being sold illegally to North Korea.

Similar envelopes were sent to the education and home affairs ministries, the national police, defense and public security investigation agencies, as well as the Agency of Natural Resources and Energy, and the National Public Safety Commission, the sources said.

Police were first notified of the case by the Education Ministry, which received the mail last Tuesday. The MPD is urging government offices to take precautions. “I am relieved to hear that the substance will have no impact (on humans),” said an official of the ministry.