Officials in Tokyo have been puzzled by an anonymous donor who has left envelopes with large sums of money in municipal rest rooms across Japan, reports the Associated Press.

Within the last few weeks, over two million yen ($16,400) in bills have been discovered in men’s bathrooms in city halls in at least 15 parts of Japan.

Handwritten letters have been found along with the money that state, “Please make use of this money for your self-enrichment,” and “One per person.”

Officials are trying to determine the identity of the donor.