TMZ obtained this never-before-seen footage ... a new angle of Wednesday's shooting as the Chiefs Super Bowl celebration wrapped, and now you can see exactly where the shooting started ... and how the injured -- including a handful of kids -- reacted.

Police have said 9 minors were among the 23 victims who suffered gunshot wounds yesterday, and here you can see a few of them hobbling away -- but heroically, also helping each other -- immediately after the barrage of gunfire outside Union Station.

You can also hear someone yelling, "It's fireworks!" -- a mistake several witnesses have said people were making amid the chaos and confusion immediately following the shots.

Remember, cops have come out and said this didn't appear to be a terroristic act, but rather, a possible dispute between parties ... which killed at least one person.

As for the investigation ... cops have only said they have 3 persons of interest in custody, and 2 of them are juveniles -- but our sources, with direct knowledge of the investigation, say cops also know there were an estimated 40 shots discharged on the scene.

We're also told cops recovered at least 3 weapons -- one of them is an AR-style pistol, and 2 others are handguns with extended magazines. It's unclear if all those weapons were fired during the shooting.

Our sources say cops believe the violence was set off by a confrontation between 2 males and one female -- and it's possible there was more than one person who opened fire in the dense crowd.

As of now, no one's been charged in the mass shooting ... and we're told the 3 persons of interest are being held in separate jails while police question them and gather more information.

Time will tell if/when more info comes to light ... but for now, we have this intel.

BTW, vigils are going on in Kansas City Thursday night to honor the victims of the shooting -- and a lot of public officials are out there ... as are citizens to pay their respects, as well as to send their prayers.