Why did Japan inspect and detain letters sent from China to

[XiaoH]

Why did Japan inspect and detain letters sent from China to the United States for over a year?
In September 1918, a registered letter was sent from Jinan via Shanghai to Japan and sent to the United States. However, it was inspected and detained during transit in Japan, and was not released for shipment to the United States until 16 months later. During World War I, Japan and the United States were both Allied countries. Why did Japan inspect mail sent to the United States and detain it for so long?