As the victims, some as young as 14 years old, recover from their injuries - none life-threatening - police are asking the public for any information or videos to help lead them to a suspect.
"It is truly a miracle that no one is dead," Madison Police Chief Shon Barnes said at a news conference Sunday morning.
He went on to explain what transpired when officers were initially called to what police believe may have been a high school graduation party at The Lux apartments on West Johnson Street at about 12:45 a.m.
Police found one teen who suffered multiple gunshot wounds bleeding in the street, and rushed in to the apartment building on Johnson Street, Barnes said.
"Officers watched as dozens of people rushed out of the building, running in panic and tripping over one another, simply trying to find safety," he said.
After running up 12 flights of stairs to reach those at the party, police said they found a chaotic scene. Authorities estimated there were several hundred people on the rooftop.
The shooting happened near the University of Wisconsin-Madison campus, but university officials said no students were known to be among those injured or involved.
Police said victims range in age between 14 and 23 years old, with some victims still in high school. Nine people suffered gunshot wounds or bullet grazes and one person was injured by broken glass at the scene, Barnes said.
"These students should have been celebrating summer vacation and not receiving medical treatment," he told reporters.
The Madison Fire Department said it took five people with gunshot wounds to the hospital by ambulance, while other victims reported to the hospital on their own. Four ambulances and two engine companies helped triage, treat and transport patients at the scene, the department said in a statement.
"Some people on scene suffered minor injuries while attempting to run away from the scene but did not request EMS," the statement added.
Authorities recovered approximately 10 shell casings at the scene and are going through apartment and public cameras to try to piece together what happened, Barnes said. With no reported motive or suspects, police are asking anyone video taken at the event to come forward.
"Whenever there's someone who's fired a gun indiscriminately like this, there's always a threat to the community," Barnes said. "We're going to work to ensure that whoever is responsible for this is brought to justice, but we can only do that with the help of our community."
Earlier this year, the Madison police and fire departments teamed up to train for how to respond to a situation like this one, Barnes said, "however, we didn't expect to put it into practice so soon."
Alder Mike Verveer, who represents downtown in Madison's Common Council, said multiple law enforcement agencies were at the scene when he arrived.
"I was lying in bed at my house ... and heard siren after siren roll by and roll through the neighborhood, and I never would have guessed that it was something as horrific as what I heard was potentially the case, which was a mass shooting," Verveer said. "I was absolutely horrified. I was absolutely horrified and the sirens didn't stop."
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WKOW and the Associated Press contributed to this report.