On Thursday, the National Toxicology Program released a report of partial findings that drew a connection between cellphone radiation and cancer. In the long-running back-and-forth over the risks of cellphones that traces back over a decade and hundreds of experiments, this study may be more than just another data point.

The National Toxicology Program’s $25 million study was conducted over multiple years and is one of the biggest experiments on this topic to date. The study involved over 7,000 animals being kept in special exposure chambers, where they were exposed to radiation frequencies for nine hours each day. Researchers found that as radiation intensity grew, so did the frequency of rare brain and heart cancers that have been previously linked to cellphone usage in humans. For reasons that the researchers still can’t identify, the linkage was particularly strong in exposed male rats, which were significantly more likely than unexposed male rats to develop a rare heart tumor and also likelier to develop a type of brain cancer.

The expansive study was one of the most “technically challenging” studies the program has ever performed, NTP associate director John Bucher said in a media conference call. “These were enormously time-consuming and expensive studies, and I can’t say that there would never be a replication of these studies, but it’s unlikely in the near future,” Bucher said.

It’s a study that former NTP associate director Chris Portier told Mother Jones is a “game-changer.” Currently, the Food and Drug Administration says on its site that “the weight of scientific evidence has not linked cellphones with any health problems, pointing to a majority of studies that have failed to find a link.” And earlier this month, a national study by University of Sydney researchers found that cellphones didn’t contribute to brain cancer rates. That study compared national cancer data in Australia with trend analyses of mobile use, and it found no correlation except in the oldest demographic aged 70-84.

Most studies using animals to address the question have been inconclusive. But it’ll be a while before the NTP study findings are expanded upon and considered by public health agencies: the NTP won’t release the remainder of the report until 2017. According to Bucher, the program released this portion of the report early because of the implications it could have for discussions about cellphone safety.

Of course, like any scientist, Bucher isn’t sounding alarms. He said Friday that he is still using his cellphone as he always has and that it’s not clear that the study is relevant to human cellphone usage. One thing is for sure: the “weight of scientific evidence” not linking cellphones to cancer that the FDA cites has just encountered a pesky counterweight.