A federal judge ruled Tuesday that expert witnesses can claim in court that Monsanto’s Roundup weed killer causes cancer. The decision, part of an individual plaintiff’s suit against the agricultural giant, makes it likely state judges across the country will hear thousands of separate cases, Reuters reported.

The suit reached the trial phase earlier this week, after years of pre-trial litigation. The plaintiff alleges that Monsanto has known and suppressed information that its herbicide is carcinogenic.

The lawsuit, along with on-going Freedom of Information Act (FOIA) requests, has turned up hundreds of internal documents on the matter. Earlier this year, the University of California in San Francisco added the documents to its public Chemical Industry Documents Archive.

California already considers the Roundup’s active ingredient, glyphosate, to be a carcinogen, as does the World Health Organization’s International Agency for Research on Cancer. U.S. federal agencies, however, have lagged in issuing a definitive decision. Monsanto and related industry groups claim that the link remains unproven.

Monsanto merged last month with Bayer (BAYRY), which plans to retire the Monsanto brand.