Paraquat is one of the most widely used weed killing chemicals in the world, competing with herbicides such as glyphosate, the active ingredient in Monsanto’s Roundup brand, for use in agriculture. But mounting research has linked the pesticide to the incurable brain ailment known as Parkinson’s disease.

Thousands of people with Parkinson’s are now suing Swiss agrochemical company Syngenta AG and Chevron USA. A predecessor company to Syngenta introduced paraquat to the market in the 1960s, and Syngenta remains the key maker of the pesticide. Chevron is the successor to a company that distributed paraquat in the US until 1986.

The plaintiffs claim long-term, chronic exposure to paraquat caused their illnesses, and allege the companies hid their knowledge of the risks that paraquat could cause Parkinson’s.

As part of court-ordered disclosures in the litigation, the companies provided plaintiffs’ lawyers with decades of internal records, including hand-written and typed memos, internal presentations, and emails to and from scientists and company officials around the world.

The New Lede was the first media outlet to obtain the internal corporate files and to reveal their contents in multiple stories co-published with The Guardian.