New York police are investigating a series of unusual incidents involving groups of people entering and later emerging from the city's sewer system at night, following the circulation of multiple videos recorded across Brooklyn and Queens.
Authorities said no injuries or arrests have been reported, while investigators continue to examine whether the incidents are connected.
Police sources cited by US media have suggested the individuals may have been searching underground for valuables, though officials have not confirmed a motive.
The investigation intensified after reports from several neighborhoods showed people removing manhole covers, descending into the sewer network and resurfacing hours later.
According to police, officers were first alerted late last week after a witness reported seeing eight people remove a manhole cover near McDonald Avenue and Colin Place in Brooklyn's Gravesend neighborhood.
Authorities said the group emerged roughly three hours later, with social media footage showing individuals climbing out of the sewer system and changing clothes near parked vehicles.
Later that night, police received another report of several people entering a manhole near Heyward Street and Bedford Avenue in Williamsburg.
Officials said the group resurfaced more than two and a half hours later before leaving the area by car.
Video footage showed several individuals exiting the sewer, with one person replacing the manhole cover before departing.
The recent reports follow a similar incident in Astoria, Queens, on 5 May, where residents observed several people entering the sewer system during the early morning hours.
Surveillance footage showed individuals carrying flashlights and wearing protective gear.
The New York Police Department said its Emergency Services Unit inspected sections of the sewer network to ensure there was no threat to public safety and found nothing suspicious.
The city's Department of Environmental Protection also reported no damage to sewer infrastructure.
Officials stressed that entering the sewer system is illegal and dangerous due to risks including toxic gases, flooding and unstable surfaces.
Police said the investigation remains ongoing as authorities work to determine whether the incidents are linked and identify those involved.