“But no explosives or hazardous substances were found,” a police spokesman told AFP, without giving further details.
Public broadcaster NHK said the box contained a printout of a Change.org petition seeking a lenient sentence for Yamagami.
The petition has amassed 13,000 signatures since last year, and its organiser admitted to NHK that they had sent the document to the court.
“I sent the signatures in the hope that they would be seen by as many interested parties as possible,” the broadcaster quoted the person as saying, without identifying them.
“As they were thick and it was raining, I put them in cardboard boxes … There was never any intention to obstruct justice.”