The New York Police Department is investigating several incidents in which groups of people have been seen entering and emerging from sewer maintenance holes across Brooklyn and Queens late at night. The NYPD told NBC in a statement on Monday that its Emergency Services Unit entered the sewer system to ensure no threat to the public existed and found nothing left behind by the individuals. The city’s Environmental Protection Department also inspected the system and reported no damage to equipment.

No arrests or injuries have been reported in connection with the incidents.

A senior law enforcement official told NBC that one theory investigators are exploring is that the individuals were searching for valuables that find their way into the sewer system.

Police were first alerted to the unusual activity on Thursday, May 28, at about 11 p.m., according to NBC New York. A witness reported seeing eight people remove a maintenance hole cover near McDonald Avenue and Colin Place in the Gravesend neighborhood of Brooklyn. The NYPD said the individuals were seen emerging from the maintenance hole roughly three hours later. Social media video showed eight people climbing out of the sewer system and changing clothes beside several parked cars.

About two hours after the initial report in Gravesend, police received another call about several people entering a maintenance hole near Heyward Street and Bedford Avenue in the Williamsburg section of Brooklyn, NBC New York reported. Authorities said the group resurfaced more than two and a half hours later before getting into a car and driving away. Video obtained by Williamsburg 365 showed seven people climbing out of the maintenance hole, with the last person closing the cover behind them.

The incidents are reminiscent of a similar episode on May 5, when a resident in Astoria, Queens, reported seeing several people open a maintenance hole cover at about 2 a.m. near 20th Avenue and 36th Street. Witness Aki Jakupovic told NBC New York that he observed “three random guys walking around in a strange suit, open the sewer, go in like Ninja Turtles.” Surveillance footage obtained by NBC New York showed the men carrying flashlights and wearing hip waders.

“I was looking at them, they were looking at me, you know, I could tell they were up to no good,” Jakupovic told NBC New York. “They went in there, closed the cover, like, you know, they were never here.”

It remains unclear whether the incidents are connected. The incidents come after MSI previously reported on a separate case in which a woman died after falling into an uncovered maintenance hole in Midtown Manhattan.