At least one of the envelopes, dated June 6, contained a message warning that radioactive materials were being sent from Japan to North Korea, a police official said. Another government official reported getting a letter containing a sand-like substance.
Prime Minister Yoshiro Mori's residence received one of the envelopes last Thursday, said Chief Cabinet Secretary Mikio Aoki. The envelope contained 3 grams of powder of about 1 micro-sievert of radioactivity -- too little to harm humans, he said.
The Science and Technology agency said the average person is exposed to about 1,000 micro-sieverts of radioactivity a year.
Kyodo News agency reported that an initial examination indicated the substance may be ground monazite, a mineral containing thorium, a nuclear fuel material. The envelopes were postmarked in Tokyo.
Nine government offices received the mysterious mail, and the government has warned ministries and agencies not to accept packages without the name of the sender, said Kazuhiko Koshikawa, a Mori spokesman.
It was not immediately clear if the mailings were meant to injure anyone. The threat brought memories of the Aum Shinri Kyo cult's 1995 nerve gas attack on subways in Tokyo's central government district, which killed 12.
The Education Ministry received an envelope containing the mysterious substance with a letter warning that "radioactive substance is being sent to North Korea and police should investigate because it is dangerous," said Hajime Kajiwara, an official of the Kojimachi police station near the ministry.
Kazunobu Asada, an Education Ministry spokesman, said the envelope contained "a very small amount of a sand-like substance."
Similar envelopes were sent to the Home Affairs Ministry, 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, Kyodo reported.
Yoshinori Inoue, an official at the Home Affairs Ministry, said the ministry received a letter dated June 6 but did not accept it because the sender's name was not on the envelope.
Also Monday, a package bomb exploded at a lawyers' office in Tokyo, slightly injuring a woman's right hand, police said, refusing the release further details. The office was located near major government offices in Tokyo.
By Shigeyoshi Kimura