A study released Friday found an increased risk of certain cancers in animals exposed to cell phone radiation, a conclusion that could reignite concerns over the safety of the widely used devices.

However, some scientists expressed serious concerns about the study, noting inconsistencies in its results. Even officials at the National Toxicology Program, which conducted the research, said the report fails to provide the clear answers many seek.

Researchers found small increases in rare cancers in the brains and hearts of male rats exposed to nearly constant, high doses of radiation from cell phones, compared to rats that weren’t exposed. There was no increase in cancer among exposed female rats, according to the study, which represents “partial findings” of a larger project that includes experiments in mice.

The rats were exposed to “whole body” cell phone radiation for a total of nine hours a day for two years. Between 2.2% to 3.3% developed malignant gliomas, a type of brain tumor. Between 1.1% and 6.6% developed a type of tumor called a schwannoma in the heart.

None of the rats in the control group, which wasn’t exposed to radiation, developed either type of tumor. Yet, in a surprising finding, those animals lived shorter lives than the ones exposed to the cell phone radiation.

The low-frequency radiation emitted by cell phones has long raised concerns that the devices may cause brain tumors, especially near side of the head where people hold their phones. The International Agency for Research on Cancer, part of the World Health Organization, classified mobile phone use “as a possible carcinogen” in 2011.

However, the radiation given off by cell phones is fundamentally different than the ionizing kind known to cause cancer. Unlike the kind given off by atomic bombs, the non-ionizing radiation emitted by phones is too weak to damage DNA. That’s led many scientists to dismiss the notion that cell phones could cause cancer.

Some cancer experts predict the new study will change that thinking. The study represents a “paradigm shift in our understanding of radiation and cancer risk,” Otis Brawley, chief medical officer at the American Cancer Society, said in a statement.

“The findings are unexpected; we wouldn’t reasonably expect non-ionizing radiation to cause these tumors,” said Brawley, who wasn’t involved in the research. “This is a striking example of why serious study is so important in evaluating cancer risk.”

The CTIA, which represents the cell phone industry, said the study’s findings conflict with other medical evidence. In a statement, the group noted there’s been no increase in brain cancer since the 1980s, when people first began using cell phones.

Authors of the study acknowledged its limitations.

Cancer rates among rats in the control group were far below what’s typical in lab animals, which could skew the results, authors noted in the study. The unexposed rats also may not have lived long enough to develop brain tumors, they noted.

The unusually low brain tumor rates among unexposed rats could also create the illusion of a difference in cancer risk that doesn’t actually exist.

Some scientists rejected the study’s conclusions. In written comments published with the study, Michael Lauer, a researcher with the National Institutes of Health’s office of extramural research, said the study may have been too small to produce reliable results and doesn’t prove that cell phones cause cancer.

John Bucher, associate director of the National Toxicology Program, said it’s not clear what, if anything, the animal study reveals about cell phone use in people. People who hold cell phones near their ears probably get the greatest radiation exposure on one side of their skull. The animals in this study were exposed to cell phone radiation across their entire body.

“It may have relevance,” Bucher said. “It may have no relevance.”

Brawley noted rats in the study were exposed to extremely high signal strengths that were “near but below levels that would cause animal tissue to heat up.”

“Additional research will be needed to translate effects at these high doses to what might be expected at the much lower doses received by typical or even high-end cell phone users,” Brawley said. “Cell phone technology continues to evolve, and with each new generation, transmission strengths have declined and with it radio frequency exposures.”