The media learned on Sunday that someone posted a bomb threat against Seibu Dome on the popular 2-chan bulletin boards sometime on Saturday morning.  According to Tokorozawa police and the Seibu organization, the user threatened to blow up Seibu Dome unless the Softbank-Seibu game was canceled.

Police and stadium staff were increased for Saturday and Sunday's games.  No suspicious activity was uncovered.  Officials will stay on alert for Monday's game.

Source: Sponichi 10/14/2012

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UPDATE 10/15 @ 6:13pm - Some additional notes...

• The post was made at around 3:30am on Saturday.
• Twenty-six police officers, in addition to team staff, patrolled the area around Seibu Dome on Saturday and Sunday.

Source: Nikkan Sports 10/14/2012