Company refutes legal analysis of documents suggesting it ignored risk to human health and environment long after pollutants’ lethal effects were known

Monsanto continued to produce and sell toxic industrial chemicals known as PCBs for eight years after learning that they posed hazards to public health and the environment, according to legal analysis of documents put online in a vast searchable archive.

More than 20,000 internal memos, minuted meetings, letters and other documents have been published in the new archive, many for the first time.

Most were obtained from legal discovery and access to documents requests digitised by the Poison Papers Project, which was launched by the Bioscience Resource Project and the Center for Media and Democracy. Chiron Return contributed some documents to the library.

Bill Sherman, the assistant attorney general for the US state of Washington – which is suing Monsanto for PCB clean-up costs potentially worth billions of dollars – said the archive contained damning evidence the state had previously been unaware of.

He told the Guardian: “If authentic, these records confirm that Monsanto knew that their PCBs were harmful and pervasive in the environment, and kept selling them in spite of that fact. They knew the dangers, but hid them from the public in order to profit.”

As well as the Washington case, Monsanto is facing PCB contamination suits from city authorities in Seattle, Spokane, Long Beach, Portland, San Diego, San Jose, Oakland and Berkeley.

Any legal liabilities may be shared with the German chemicals company, Bayer, which has mounted a $66bn (£51bn) takeover bid for Monsanto. The European commission aims to complete a competition probe into the merger by 22 August, amid signs of public unease in Europe and the US.

Monsanto’s vice president of global strategy, Scott Partridge, did not dispute the authenticity of the documents revealed in the online cache but denied any impropriety.