A message included in envelopes containing small amounts of a radioactive substance mailed earlier last week to government offices alleged that an Education Ministry foundation is smuggling uranium to North Korea, police sources said Tuesday.

The envelopes, postmarked in Tokyo and containing radioactive powder believed to be monazite, were mailed to 10 government offices, including Prime Minister Yoshiro Mori’s Official Residence, the sources said.

During questioning by police, the president of the foundation, which is based in Tokyo’s Taito Ward, admitted storing a large amount of monazite in the Kanto region, including Kumagaya, Saitama Prefecture, and Tatsuno, Nagano Prefecture, the sources said.

While the identity of the envelopes’ sender remains unknown, police suspect that someone who knows the president sent them, the sources said. The name of the president was mentioned in the message.

Police and Science and Technology Agency officials have already located the stored monazite, but no harmful levels of radioactivity were detected at the two sites, they said.

Monazite contains Thorium, an element used in nuclear fuels.

The president said that he imported some 50 tons of monazite from Thailand about 20 years ago to sell to operators of hot springs that contain radium, and that the remaining 15 tons of monazite have been in secure storage.

The president said that a man who contacted the foundation six months ago was asking the foundation to sell him some monazite, but that he declined because the man seemed suspicious.

“I have no idea who sent the envelopes,” the president said.

The Public Security Bureau of the Metropolitan Police Department is investigating how the executive imported and stored the materials, and whether he violated a law regulating nuclear reactors and nuclear-related facilities, the sources said.

Each envelope contained several grams of the radioactive substance, but their radioactivity was estimated to be less than 1 microsievert, police said.