(CHICAGO) -- Authorities believe Monday's mass shooting at a Fourth of July parade in Highland Park, Illinois, had been planned for weeks, and they say more than 70 rounds were fired from the gunman's high-powered rifle, which was similar to an AR-15.
The 21-year-old suspect, Robert "Bobby" Crimo III, is accused of opening fire from a roof of a business, which he accessed from a fire escape ladder, police said.
Six people were killed and over 30 were injured in the suburban Chicago shooting.
Police said Crimo wore women’s clothing during the shooting to apparently allow him to hide his facial tattoos and blend in with the crowd to flee.
"Following the attack Crimo exited the roof, he dropped his rifle and he blended in with the crowd and he escaped," police said Tuesday. "He walked to his mother's home who lived in the area and he blended right in with everybody else."
It appears Crimo bought the high-powered rifle legally in Illinois, police said.
No motive is known, police said. When asked by reporters if the gunman targeted anyone specifically, police said the "shooting appears to be completely random."
Crimo is answering questions from investigators and has made statements taking responsibility for the attack, according to multiple law enforcement sources.
Crimo -- who was apprehended Monday evening after an hours-long manhunt -- is believed to be linked to social media posts that discuss or depict acts of violence, including shooting people, a law enforcement source briefed on the case told ABC News.
Online posts include a video showing what appears to be a portion of the same parade route where the shooting took place.
In a video posted over a year ago to his YouTube page, Crimo is shown in what appears to be a depiction of the aftermath of a school shooting.
Crimo had been living with his uncle, Paul Crimo, but the two barely interacted beyond exchanging hellos, Paul Crimo told ABC News.
Paul Crimo said he last spoke to his nephew the evening before the shooting and said he was shocked to learn about his alleged involvement.
He described his nephew as quiet and respectful and said the 21-year-old never mentioned firearms.
The mass shooting broke out when Highland Park's parade was about three-quarters of the way through Monday morning, authorities said.
Revelers fled in panic, leaving behind empty strollers, overturned chairs and half-eaten sandwiches.
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."
Dr. David Baum was watching his grandson, daughter and son-in-law march in the parade when the gunfire began.
"Bodies were horribly, horribly, horribly injured from, you know, guns and bullets that were made for war -- not for parades," Baum said of some of the victims.
"The paramedics went quickly and assessed the damages -- saw bodies that were blown apart and put a blanket over them quickly. And then went on to try and help other people," he told ABC News. “These are injuries that nobody should have to see."
Five people -- all adults -- died at the scene and a sixth adult died at a hospital, officials said.
Crimo was at large for hours after the shooting. After police released an image of Crimo and his car Monday evening, he was spotted driving and led police on a brief pursuit, authorities said.
He was stopped at U.S. Highway 41 and Westleigh Road in Lake Forest, Illinois, where he surrendered, according to police.
A second weapon, also purchased legally by Crimo, was found in the car, police said.
President Joe Biden said in a statement that he "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."
Vice President Kamala Harris said in a statement, "On what should be a celebratory day with family and friends, we are grieving the lives that were taken in another act of senseless gun violence."
"More should be done to address gun violence in our country," she said. "President Biden recently signed into law the first major bipartisan gun reform legislation in almost 30 years -- and we will continue fighting to end this senseless violence."
An impassioned Illinois Gov. JB Pritzker said, "It is devastating that a celebration of America was ripped apart by our uniquely American plague. A day dedicated to freedom has put into stark relief the one freedom we as a nation refuse to uphold: the freedom of our fellow citizens to live without the daily fear of gun violence."
"I'm furious that yet more innocent lives were taken by gun violence. I'm furious that their loved ones are forever broken by what took place today. I'm furious that children and their families have been traumatized," he said. "While we celebrate the Fourth of July just once a year, mass shootings have become our weekly -- yes, weekly -- American tradition. There are going to be people who say that today is not the day that now is not the time, to talk about guns. I’m telling you there is no better day and no better time than right here and right now.”
"Our founders carried muskets, not assault weapons. And I don’t think a single one of them would have said that you have a Constitutional right to an assault weapon with a high-capacity magazine -- or that that is more important than the right of the people who attended this parade today to live," the governor said.
Representatives of the gun reform group March For Our Lives, founded by survivors of the 2018 high school mass shooting in Parkland, Florida, said in a statement, "Just three weeks ago, young people organized a March For Our Lives in Highland Park, along with communities across the country."
"We are grieving for the horrific loss of life in Highland Park, and the carnage brought on by a high-powered rifle," they said. "We wish eternal peace for those who were murdered, and we will fight like hell for the living."
New Jersey Gov. Phil Murphy is among the leaders reacting to the nation's latest mass shooting, tweeting, "Not even a parade on the Fourth of July celebrating our nation’s independence is immune from our nation’s gun violence epidemic. Tomorrow, I will sign seven sweeping commonsense gun safety bills into law. We cannot wait."
Secretary of Homeland Security Alejandro Mayorkas said in a statement that the U.S. must "address the epidemic of targeted gun violence, including the development and implementation of new community-based models of prevention and intervention."
"The Department of Homeland Security will redouble its work in this critical area and help lead the effort to prevent violence," he vowed.
This is a developing story. Please check back for updates.
ABC News' Josh Margolin, Pierre Thomas, Aaron Katersky, Alex Perez, Jack Date, Will Steakin, Jeff Cook, Will McDuffie and Caroline Guthrie contributed to this report.
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