A Russian cosmonaut has claimed that living bacteria found on the outside of the International Space Station came from outer space.

Cosmonaut Anton Shkaplerov told Russian news agency TASS that the ISS crew used cotton swabs to take samples from the external surface of the ISS during their spacewalks, particularly from areas where fuel wastes accumulated after being discharged from the engines, as well as obscure surface areas of the station where activity is low. The samples were then sent back to Earth for testing. Shkaplerov claims that the bacteria was not present during the module’s launch and therefore come from space, not Earth. Shkaplerov also stated that the extraterrestrial bacteria is not dangerous, and also that it is distinct from other terrestrial bacteria also discovered on the ISS exterior. (That bacteria likely arrived accidentally when tablet PCs were transferred up to the space station.)

However, others aren’t so sure that the newly found bacteria is from extraterrestrial sources. Slate points out that the upper reaches of Earth’s atmosphere host many undiscovered forms of life, such as highly adaptable bacteria that can withstand low-pressure, low-oxygen altitudes in the air as well as intense UV radiation — and perhaps this tough bacteria could theoretically find a way to survive outside the atmosphere. An unknown microbe also could have attached itself to one of the hundreds of spacecrafts flown up toward the ISS.

This also isn’t the first time cosmonauts have said they found something unusual on the space station. A 2014 claim stated that they found sea plankton clinging to the ISS. Meanwhile, NASA said it hadn’t heard from the Russians about that one.

Experts also caution that information coming out of Russia and TASS could be “fake news.”

NASA has not responded to the latest discovery.