A group calling itself a revolutionary army claimed responsibility Friday for Tuesday's fire in a lot adjacent to the office of a group that authored a controversial history textbook.
A letter dated Wednesday received by news organizations says the senders "launched a revolutionary firebomb attack of anger" against the Japanese Society for History Textbook Reform, whose members wrote the textbook vehemently slammed by China and South Korea for glossing over Japan's wartime atrocities.
Police suspect the senders are an antimainstream faction of a leftist radical group opposed to the textbook.
The letter also says the attack was retaliation against the content of the text and Tuesday's decision by the Tokyo Metropolitan Education Board to adopt the textbook for use from April at some public junior high schools for physically and mentally disabled children.
The fire, which started around 11:50 p.m. Tuesday, scorched the window frame of the society's building in Tokyo's Bunkyo Ward but caused no other damage.
Police said the case was a guerrilla act directed at the society and they suspect radicals placed an incendiary device in the parking lot to start the fire.