Authorities in New York City are investigating a strange series of incidents involving groups of people emerging from manholes after multiple videos of the activity circulated online.

The NYPD said its Emergency Services Unit searched the sewer system as a precaution and found no evidence of danger or damage, and no arrests or injuries have been reported.

The incidents were first reported in Brooklyn’s Gravesend and Williamsburg neighborhoods, where groups were seen entering maintenance holes late at night and reappearing hours later before leaving in vehicles.

Investigators are considering the possibility that the individuals were searching the sewer system for valuables, The Guardian has reported.

Similar reports from earlier in the year and elsewhere in the city suggest a potential pattern, though it remains unclear whether the cases are connected.