Tokyo's Nerima Ward office has stepped up security measures in response to a message on an Internet message board threatening to detonate a bomb at the building Monday, but no explosive has been found so far, sources said Sunday.

The sources at the ward and the Nerima Police Station of the Metropolitan Police Department said a further search would be conducted before the ward office opens its doors to visitors Monday and the number of security guards will be increased, while the police are seeking to identify the individual who posted the anonymous threat on the message board.

The message, which is believed to have been posted in the early hours of Sunday, contained criticism against the police and challenged them by saying, "catch me if you can." It specifically cited a legal firm in Tokyo's Minato Ward and the Nerima Ward office as bombing targets.

Police are investigating the case, alleging it constitutes the forcible obstruction of business.

It is not known what specific message board was used for the threat.

The latest bomb scare follows a mailed bomb threat sent to the Adachi Ward office on June 1. The police are still investigating the case, in which a message in an envelope said a bomb had been planted in the building, again claiming the threat constituted the forced obstruction of business.