Eleven hospitals in Tokyo have received letters this month threatening bomb attacks, but no explosives have been found at any of the facilities, a news report said Monday.
The reported threats come as security has been tightened amid a campaign for parliamentary elections. An anonymous bomb threat last week forced the evacuation of an airliner in northern Japan.
The letters offered no details, only saying that the bomb attacks would come within 45 days, Kyodo News agency reported, citing unidentified officials at Tokyo Metropolitan Police.
The letters made no demands, and it was unclear whether the object of the attacks would be the hospitals or other targets, the report said.
Police would not confirm the account. But one of the recipients, Keio University Hospital, got a Sept. 1 letter warned of a "large-scale terrorist attack," said spokesman Nobuo Kuroda, reports the AP.