The attorney general of Missouri will investigate a “day care fight club” in St. Louis where pint-size pugilists were caught on camera punching each other in a basement classroom, officials announced Wednesday.

Missouri Attorney General Eric Schmitt said his office served a civil investigative demand on Tuesday to the Adventure Learning Center on Gravois Avenue in connection with a December 2016 video showing workers encouraging youngsters as young as 3 years old to fight each other.

“Children are some of the most vulnerable members of our society, and any attempts to harm them in any way will not be tolerated,” Schmitt said in a statement. “My job is to protect all 6 million Missourians, and my office will continue to investigate and bring to justice those who seek to harm others.”

The video — first obtained by KTVI — shows workers Mickala Guliford, 28, and Tena Dailey, 22, goading two young boys to fight each other while using oversize, foam-rubber “Incredible Hulk” hands. The brawls left one 4-year-old with a black eye, red marks and facial swelling, the St. Louis Post-Dispatch reported.

Both Guliford and Dailey were arrested and lost their jobs, but charges of felony first-degree endangering the welfare of a child creating a substantial risk were dropped earlier this month due to a lack of evidence, according to the newspaper.

Nicole Merseal, the mother of a 10-year-old boy who recorded the video, has sued the day care facility for $25,000 in damages after sharing the footage with reporters. She claims her older son witnessed the fighting via a window from an adjacent classroom and pounded on the glass after seeing his younger brother in tears.

Guliford, meanwhile, has admitted encouraging the children to get physical with each other, but dismissed it as a “stress release exercise.”

A civil investigative demand compels entities to produce documents relating to an investigation launched by the attorney general. The probe is separate from an ongoing investigation by St. Louis Circuit Attorney Kim Gardner, Schmitt said.

A woman at the day care facility who refused to identify herself declined comment before abruptly hanging up when reached Thursday by The Post.