Astronomers hunting for signs of intelligent alien life in the universe have recorded 15 mysterious radio signals coming from a dwarf galaxy three billion light years away.

Using the Green Bank Telescope in West Virginia, scientists with the Breakthrough Listen initiative—a massive project dedicated to finding signs of intelligent alien life—recorded 15 repeating fast radio bursts (FRBs) on August 26. The discovery was announced as an Astronomer’s Telegram and will be described in further detail in a forthcoming scientific article, according to a statement from Breakthrough Listen.

FRBs last just a few milliseconds and appear to be coming from deep space. Because FRBs have an extremely short duration, and because scientists usually find them in data only after the event has taken place, pinpointing their origin has not been possible.

Since their discovery over 15 years ago, almost two dozen FRBs have been recorded. Most often, they are one-off events, but in 2016 scientists announced in the journal Nature that they had found a repeating radio signal—FRB 121102. By monitoring and tracking this repeating burst, they were able to trace it back to a dwarf galaxy 3 billion light-years away. Still, the source remained elusive. Nothing that we know of in that region of space could be producing these signals.

Now the Breakthrough Listen team has detected 15 more busts coming from FRB 121102. Vishal Gajjar, a postdoctoral researcher at the University of California, Berkeley, where Breakthrough Listen is based, observed the new bursts during a monitoring effort run. Over five hours of observation, he and his colleagues collected 400 terabytes of data over the 4 to 8 GHz frequency band, or C-band, which is mostly used for satellite communications transmissions.

Analysis of the data revealed the new repeating pulses and showed that the source is in a “heightened activity state,” the Astronomers Telegram said. “Follow-on observations are encouraged, particularly at higher radio frequencies,” the team added.

Initial results indicate that FRBs emit at higher frequencies than previously observed. This discovery that should help scientists determine the source producing the bursts.

Several explanations for FRBs have been suggested. One is a cataclysmic event, such as a neutron star collapsing into a black hole. But such an event would produce only one burst and therefore does not explain the repeating pattern of FRB 121102. Another possible explanation is that they are coming from a young, highly magnetized neutron star, but so far nothing like this has been detected in this region of space.

Despite widespread speculation, the possibility of the signals coming from an advanced alien civilization has been largely ruled out.