An envelope containing a box-cutter blade and a letter opposing the restart of a nuclear power plant was mailed to industry minister Yuko Obuchi late last month, investigative sources said Monday.

The anonymous letter opposed the restart of the Sendai nuclear power plant in Kagoshima Prefecture, the closest to restarting after the Nuclear Regulatory Agency recently declared it meets the safety standards introduced last year in response to the 2011 Fukushima nuclear crisis.

In a recent written message to communities near the Sendai plant, Obuchi, who became minister for economy, trade and industry last month, indicated the government will work to restart the nuclear facility.

A name was written on the envelope, which was postmarked in Japan, but the real sender remains unknown.