For years, scientists at Monsanto Co. worked closely with outside researchers on studies that concluded its Roundup weedkiller was safe.

That collaboration is now one of the biggest liabilities for the world’s most widely used herbicide and its new owner, Bayer AG , which faces mounting lawsuits alleging a cancer link to Roundup.

Plaintiffs’ attorneys are putting Monsanto’s ties to the scientific community at the center of a series of high-stakes suits against Bayer. Since the German company acquired Monsanto last June, two juries in California have sided with plaintiffs who have lymphoma and blamed the herbicide for their disease. Bayer’s shares have fallen roughly 35% since the first verdict.

In both cases, plaintiffs’ attorneys argued that Monsanto’s influence on outside studies of Roundup’s active ingredient tainted the safety research. The attorneys obtained certain Monsanto emails showing outside scientists asking the company’s scientists to review their manuscript drafts, and Monsanto scientists suggesting edits.

Gary Kitahata, a member of a jury that ordered Bayer to pay $289.2 million to a former California groundskeeper with non-Hodgkin lymphoma last August, said Monsanto’s interaction with outside researchers played an important role in jurors’ deliberations. He recalled being struck by emails allegedly dealing with “things like ghostwriting, influencing scientific studies that were done.” A judge later reduced the award, which Bayer is appealing, to $78.5 million.