Apr. 23, 2015 - Updated 04:12 UTC
Tokyo police investigating a drone found on top of the Prime Minister's Office say nobody had been up there for the last month.
Police are checking surveillance cameras in the vicinity on Thursday to determine when the drone landed. They are also analyzing the contents of a container mounted on the device, which contained radioactive cesium.
The drone was found on the rooftop on Wednesday.Police suspect it was intentionally flown over the Prime Minister's Office.
They said staffers at the Office found it when they went to the rooftop as part of training for new hires.
Investigators told reporters the heliport on the roof was last used on March 22nd and that the drone was not there then. They say nobody had been up on the roof since.
Police are stepping up surveillance of an area of several hundred meters around the Prime Minister's Office and the National Diet Building.
Officers are monitoring suspicious flying objects and drone operators, while intensifying stop-and-searches.
They are asking the operators of nearby buildings to lock entrances to rooftops and patrol the buildings thoroughly.