Six people were killed and about 24 others were seriously hurt in a mass shooting at a Fourth of July parade in the Chicago suburb of Highland Park, Illinois, on Monday, according to officials.

Highland Park police are still searching for the gunman, who they described as a man between the ages of 18 and 20.

It appears the gunman fired from a roof where a high-powered rifle was recovered, police said.

Five people — all adults — died at the scene and a sixth victim died at a hospital, said Lake County Coroner Jennifer Banek.

The NorthShore University Health System said it has a total of 31 patients; most suffered gunshot wounds and a few were hurt in the chaos. At least one child is critically injured, Highland Park Fire Chief Joe Schrage said.

Police are describing the shooting as a “random act of violence.”

The parade was about three-quarters of the way through when the shooting broke out, authorities said. Revelers fled in panic, leaving behind empty strollers, overturned chairs and half-eaten sandwiches.

Police ran toward the shots but the gunman had already fled, Lake County Sheriff Deputy Chief Christopher Covelli said at a news conference.

When the gunfire erupted, parade-goer Zoe Nicole Pawelczak grabbed her dad and started running through the sea of people.

“I saw multiple people slaughtered,” she told ABC News.

“Everybody is crying. We ended up making it behind a corner and we hid behind a dumpster. This man was there with his two very young children and he had put them in the dumpster for safety,” she said.

Pawelczak said the man wanted to leave to find his other son, and asked her to watch the two children in the dumpster.

“So I watched his kids for him,” she said. “They were like, ‘What’s going on?’ And I was like, ‘It’s just fireworks, it’s OK,’ just trying to keep them calm.”

Jon Will was at the parade with his 97-year-old mother, who uses a wheelchair. The family hustled to push his mother to an alley to hide out, he told ABC News.

“This doesn’t happen in a little town like this. Just unbelievable,” he told ABC News. “It’s scary. I don’t know what this world is coming to. It’s ridiculous.”

Rep. Brad Schneider, D-Ill., who was at the parade, tweeted that he’s committing himself “to do everything I can to make our children, our towns, our nation safer. Enough is enough!”

Police are asking anyone who was at the parade to review their videos and share them with authorities.

The Bureau of Alcohol, Tobacco, Firearms and Explosives has agents on scene and is conducting an urgent trace on the recovered gun, according to a law enforcement source.

The neighboring suburb of Evanston has canceled its own Fourth of July parade in the wake of the shooting, Evanston police said.

President Joe Biden said in a statement that he’s “surged Federal law enforcement to assist in the urgent search for the shooter.”

“Members of the community should follow guidance from leadership on the ground, and I will monitor closely as we learn more about those whose lives have been lost and pray for those who are in the hospital with grievous injuries,” Biden said.

He noted that he recently signed into law the most significant gun control legislation in decades, adding, “But there is much more work to do, and I’m not going to give up fighting the epidemic of gun violence.”

In an impassioned statement Illinois Gov. JB Pritzker said, “There are no words for the kind of monster who lives in wait and fires into a crowd of families with children celebrating a holiday with their community. There are no words for the kind of evil that robs our neighbors of their hopes, their dreams, their futures.”

“Prayers alone will not put a stop to the terror of rampant gun violence in our country,” the governor wrote. “We must — and we will — end this plague of gun violence.”

The Chicago White Sox announced plans to hold a moment of silence before Monday night’s game against the Minnesota Twins, and have canceled the postgame fireworks.

Law enforcement has long been concerned about gunmen firing from elevated positions, which police say can give them a strategic advantage.

The deadliest example of that is the 2017 Las Vegas massacre, during which the shooter took up a window position in the Mandalay Bay hotel. Fifty-nine people were killed in what became the deadliest mass shooting in modern U.S. history.

In the wake of the Vegas massacre, law enforcement around the country placed a greater emphasis on securing elevated locations surrounding public events, but police have acknowledged how daunting a task it is to secure all such positions.

This is a developing story. Please check back for updates.

ABC News’ Alex Perez, Jack Date and Josh Margolin contributed to this report.