The growing use of cellphones has raised alarms over the last few years, with claims — sometimes unsubstantiated and often rubbished — that radio frequencies used by our now-ubiquitous companions are carcinogenic, or cancer-causing. However, partial findings from a U.S. government study, released Thursday, could provide some credence to those claims, as they draw a link between radio frequency radiation and two types of cancer.

The U.S. Food and Drug Administration (FDA) asked the National Toxicology Program (NTP) to conduct toxicology and carcinogenicity tests linked to exposure to “radio frequency radiation emitted by wireless communication devices, especially cellphones.” To test for its potential carcinogenicity on humans, rats were exposed to radiation for periods of up to two years.

Findings of the study “appear to support the International Agency for Research on Cancer conclusions regarding the possible carcinogenic potential” of radiation from radiofrequencies used in mobile communication. In 2013, IARC had classified such radiation as a possible human carcinogen, and that study, along with the extremely large number of people who use cellphones, prompted the FDA to commission the NTP study.

The study, carried out at IIT Research Institute in Chicago, used both CDMA and GSM radio frequencies, the two types currently used in U.S. wireless networks. Different groups of test subjects were exposed different levels of radiation while still inside the uteruses of pregnant female rats, and for up to a period of 106 weeks, during which time they were exposed for about 9 hours a day, every day.

Male rats exposed to larger amounts of CDMA radiation showed “a statistically significant positive trend in the incidence of malignant glioma” in the brain, while “a low incidence of malignant glioma” was seen in all groups of male rats exposed to GSM radiation. Female rats, on the other hand, showed fewer instances of glioma in both GSM and CDMA radiations, but incidences linked to CDMA radiation were caused by lower radiation doses than for male rats.

Schwannoma of the heart, a rare type of cancer, was the other tumor observed in the rats exposed to radiation. “Cardiac schwannomas were observed in male rats in all exposed groups of both” GSM and CDMA radiations of varying levels. Exposed female rats, in contrast, showed far fewer instances of schwannoma.

In its conclusion, the study says its results “are limited to select findings of concern in the brain and heart and do not represent a complete reporting of all findings” from studies on cellphone radiation.

Curiously, the study also found that rats exposed to radiation showed higher incidents of survival after the two-year-long study, compared to groups of rats who served as control groups — they were part of the experiment but not exposed to radiation.