WASHINGTON, D.C. — USDA has concluded its months-long investigation into last year’s “mystery seed” case, during which thousands of people across the U.S. received unsolicited seed packages in the mail.

The Agriculture Department said that after a detailed investigation, it found no evidence that someone was trying to harm U.S. agriculture. The report concluded the seeds were likely part of an internet scam to help a seller boost its search results.

The investigation also prompted the agency to create new guidelines to prevent such events in the future.

Last summer, thousands of Americans nationwide received mysterious seed packages, labeled as though they were from China, Uzbekistan, the Solomon Islands, the United Arab Emirates, Kyrgyzstan and elsewhere.

The mass mailing sparked concern about foreign seeds. State and federal agriculture departments issued warnings to citizens not to plant the seeds, saying the packages could potentially contain invasive species, pathogens or pests.

“We are thankful to the public for reporting the seeds and getting them to USDA officials so that we could investigate them and avoid introducing foreign pests into our environment,” Osama El-Lissy, plant protection and quarantine program deputy, said in a statement.

A few months into the investigation, USDA concluded that the seed samples were benign, including varieties like rosemary, cabbage, mustard and basil.

Nevertheless, USDA’s Animal and Plant Health Inspection Service, or APHIS, concluded the seed shipments were illegal because they entered the U.S. without a permit or phytosanitary certificate.

Agency spokeswoman Cecilia Sequeira told the Capital Press last summer that the packages were probably part of a global “brushing scam,” generating fake sales to boost a seller’s legitimacy. But the agency did not issue that final conclusion officially until this month, after further probing and analysis.

Last year’s scare prompted a few major changes that could impact seed sales in the future.

In response to the scam, online retail giant Amazon banned foreign sales of seeds in the U.S.

USDA this month, also acting in response to last year’s events, issued additional guidance to help online buyers and sellers comply with U.S. laws when they import seeds and live plants from other countries.

The new guidance explains buyer and seller responsibilities, outlines required documents, including import permits and phytosanitary certificates, provides information about which seed and plant species have import requirements and clarifies what plants are not allowed in the U.S.

“We’ve been working closely with e-commerce companies and other federal partners to stop the flow of illegal plant and seed shipments entering the country,” said El-Lissy of USDA. “This new site is a big step forward in our efforts to facilitate the safe trade of plants and seeds through the e-commerce pathway.”