3:11 PM PT -- We're now learning more about the victims from this shooting -- a majority whom are apparently children.

Multiple reports are pouring in that say of the 12 patients at a local hospital -- 11 of them are children ... and 9 of those 11 have gunshot wounds.

2:37 PM PT -- As it turns out ... an AR-style rifle may well have been used during this shooting -- something you can now see clearly in zoomed-in footage of a man getting tackled.

In the clip below -- where bystanders take a man down and pin him until cops arrive -- you see a woman in white pick up what appears to be a clear firearm ... and from the looks of it, it's a long rifle. The gun itself seems to be black -- but it's unclear if it belonged to the person who was tackled.

A lot of questions still need to be answered ... and cops are looking into it. On the face of it though -- this could've been a lot worse than it already was.

The Chiefs also addressed the tragedy ... saying, "We are truly saddened by the senseless act of violence that occurred outside of Union Station at the conclusion of today's parade and rally."

"Our hearts go out to the victims, their families, and all of Kansas City."

The team added it will continue to stay in contact with law enforcement and local officials ... and shared their appreciation to all the first responders who took action.

New video, obtained by TMZ, shows the moment a flurry of shots rang out at the Kansas City Chiefs Super Bowl victory rally.

You hear someone in the crowd scream out, "Patrick f**king Mahomes," seemingly in celebration of the Chiefs' victory. Then, in the distance, you hear a series of gunshots ... appearing to come from either an automatic or semiautomatic weapon, based on the rapid fire.

People then scramble for safety and victims fall to the ground.

At least one person was killed and authorities say 10 to 15 people were injured. None of the players, coaches or staffers were injured in the shooting.

Two people are in custody in connection with the shooting.

Several people in the crowd had eyes on someone who they believed was one of the shooters. You see them run and then tackle that person to the ground ... while another bystander seems to grab what looks like a rifle, moving it away from the scrum until police arrived on the scene.

The shooting occurred just after the parade ended.

Police do not know the motive.