Videos of men mysteriously emerging from New York City sewers drew the attention of police, who insist there’s “threat to the public,” a law enforcement source told NBC News on Monday.

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Social media posts last week appear to show groups of men, not affiliated with any public works crews, climbing out of manholes in various Brooklyn neighborhoods.

There haven’t been any immediate arrests in these apparent trespassings.

Police are pursuing their leading theory that the group is “scouring the system for valuables that get into the sewage,” a senior law enforcement official told NBC News.

“The NYPD, to make sure there was not a threat to the public, sent their highly trained Emergency Services Unit officers into the sewer system to make sure nothing nefarious had been left behind by the individuals — nothing was found — and NYC DEP which manages the system also went in and apparently found no damage to the sewer system equipment,” the official added.

In video time stamped at about 2 a.m. on Friday in Brooklyn, a man in a white shirt and shorts picks up a manhole and allows seven men to emerge. All of them appear to be wearing coveralls or protective clothing.

Then in another video shot in Brooklyn, a man in a red shirt could be seen opening a manhole cover from underneath, dangerously climbing into a street with traffic.

Six more people emerged following that first man with at least three of them carrying shovels. They all appeared to be wearing or carrying a headlamp.