SAN FRANCISCO (AP) — Evidence that Roundup weed killer can cause cancer seems "weak," but experts can still make that claim at trial, a U.S. judge ruled Tuesday.

The decision by U.S. District Judge Vince Chhabria in San Francisco allows hundreds of lawsuits against Roundup's manufacturer, Monsanto, to move forward. The lawsuits by cancer victims and their families say the agrochemical giant long knew about Roundup's cancer risk but failed to warn them.

Many government regulators have rejected a link between cancer and the active ingredient in Roundup — glyphosate. Monsanto has vehemently denied such a connection, saying hundreds of studies have established that glyphosate is safe.

Chhabria said the evidence, "viewed in its totality, seems too equivocal to support any firm conclusion that glyphosate causes" non-Hodgkin's lymphoma.

Still, the judge said he would not go as far as saying that three experts presented by attorneys for the cancer victims and their families presented "junk science" that should be excluded from trial.

Monsanto did not immediately have comment. An email to an attorney for the plaintiffs was not immediately returned.

The judge wanted to determine whether the science behind the claim that Roundup can cause non-Hodgkin's lymphoma had been properly tested and met other requirements to be considered valid.

Before issuing his ruling, Chhabria spent a week in March hearing dueling testimony from epidemiologists. He peppered them with questions about potential strengths and weaknesses of research on the cancer risk of glyphosate.

Beate Ritz, an epidemiologist at the University of California, Los Angeles, testified for the plaintiffs that her review of scientific literature led her to conclude that glyphosate and glyphosate-based compounds such as Roundup can cause non-Hodgkin's lymphoma.

Ritz said a 2017 National Institute of Health study that found no association between glyphosate and non-Hodgkin's lymphoma had major flaws.

Monsanto brought in its own expert, Lorelei Mucci, a cancer epidemiologist at the Harvard T.H. Chan School of Public Health, who praised the 2017 study and reached the opposite conclusion from Ritz.

"When you look at the body of epidemiological literature on this topic, there's no evidence of a positive association between glyphosate and NHL risk," she said of non-Hodgkin's lymphoma.

Chhabria later called the opinions of the plaintiffs' experts "shaky," though he said they still may meet scientific standards to go before a jury.

There was "at least a strong argument that the only reasonable conclusion one could draw right now is that we don't yet know" whether the herbicide is causing non-Hodgkin's lymphoma, he said.

Monsanto developed glyphosate in the 1970s, and the weed killer is now sold in more than 160 countries. Farmers in California, the most agriculturally productive state in the U.S., use it on more than 200 types of crops. Homeowners use it on their lawns and gardens.

St. Louis-based Monsanto also sells seeds that can tolerate being sprayed with glyphosate as the surrounding weeds die, ensuring another stream of business that has helped it dominate the market for genetically modified crops.

The herbicide came under increasing scrutiny after the France-based International Agency for Research on Cancer, which is part of the World Health Organization, classified it as a "probable human carcinogen" in 2015.

A flurry of lawsuits against Monsanto in federal and state courts followed, and California added glyphosate to its list of chemicals known to cause cancer. Monsanto has attacked the international research agency's opinion as an outlier.

The U.S. Environmental Protection Agency says glyphosate is safe for people when used in accordance with label directions. A draft report by the agency last year concluded the herbicide is not likely to be carcinogenic to people. The report noted that science reviews by numerous other countries had reached the same conclusion.

A federal judge in Sacramento in February blocked California from requiring that Roundup carry a label stating that it is known to cause cancer, saying the warning is misleading because almost all regulators have concluded that there is no evidence glyphosate is carcinogenic.