According to a broadcast released by the United States Department of Agriculture on Wednesday, officials from both China and the U.S. are working together in order to uncover the origin of thousands of seed packets that have been shipped from China to unsuspecting homes in the U.S. The USDA says it knows the names of some of the companies sending seeds, but needs China’s help tracking them down.

“We don’t know the background information about these companies, and that’s why we’re working with our counterparts in China to follow up on some of these senders,” Osama El-Lissy, deputy administrator for the USDA’s Animal and Plant Health Inspection Service (APHIS), said on Wednesday.

The seeds began appearing in mailboxes in the U.S. and several other countries as early as two months ago. The USDA warned receivers of the unsolicited seeds not to plant them, in case they introduced an invasive species of plant or contained bugs that could be harmful to the local biosphere. Recipients were instructed to either destroy the seeds or post them to the USDA for inspection.

Early this month, APHIS announced it had identified 14 species of seeds from among the thousands sent out to American homes. The seeds were mostly from common plants such as sage, rosemary, hibiscus, and roses. El-Lissy says the USDA has so far identified just two seeds from potentially harmful weeds as well as one lava of a common leaf beetle.

“That’s the extent of our findings, so far…Other than that, we have not found anything alarming,” said El-Lissy, who is experienced in snuffing out invasive crop pests. As director of the Texas boll weevil eradication program from 1994 through to 2000, El-Lissy led one of the largest pest eradication programs in the world. But APHIS’s mundane findings so far support the organization’s early theory that the unsolicited seed shipments were nothing more than a “brushing scam.”

Brushing is when sellers on an online marketplace, such as Amazon, falsify sales data in order to achieve higher ratings. Sending out cheap products, such as seeds, to unsuspecting customers can help boost a merchant’s sale numbers.

The U.S., like many countries, has restrictions on seed imports, because the goods can be harmful to the local ecosystem. The USDA says it is working with its counterpart in China, the Ministry of Agriculture and Rural Affairs, as well as China Post in order to “stop future shipments” of seeds.