The Metropolitan Police Department raided several locations Monday connected with a radical leftist group in connection with an arson attack last week that apparently targeted the office of a nationalist group that wrote a contentious history book.

According to sources close to the case, the sites raided included Sekisaisha, a key site of the radicals, in Taito Ward, Tokyo.

The homes of people with ties to the group were also searched, they said.

A letter dated Wednesday and sent to several news organizations says that the senders, which claimed to be a "revolutionary army," were responsible for the fire in a parking lot adjacent to the office of the Japan Society for History Textbook Reform in Tokyo's Bunkyo Ward on Aug. 7.

Police believe that there is sufficient reason to believe the leftist faction was behind the attack, prompting Monday's raids, according to the sources.

Investigators confiscated leaflets, bulletins and personal computers in their raids, they added.

The letter says the senders "launched a revolutionary firebomb attack of anger" over the junior high school history textbook penned by members of the society.

The text has been slammed by China and South Korea as whitewashing Japan's wartime atrocities.

The fire, which started late Aug. 7, began after a timed incendiary device left in the parking lot ignited, charring the window frame and walls of the building in which the society's office was located.

No one was hurt in the incident, according to police.

The fire came on the same day that the Tokyo Metropolitan Education Board decided to adopt the textbook for use at some junior high schools for physically and mentally disabled children.