New evidence suggests that as early as 2019, thein the in Highland Park, Illinois, was flagged for an attempted suicide and threats of physical violence. At the time, 16 knives, a dagger and a sword were confiscated, but local investigators ran into roadblocks.
"There were no complaints that were signed by any of the victims," Chris Covelli, spokesman for the Lake County Major Crimes Task Force, said Tuesday.
The suspect's now removed videos also glorified mass shootings to his digital followers. Scott Sweetow, a former FBI and ATF investigator, told CBS News there is a digital tsunami coming at investigators.
"One of the biggest challenges is, how do you go through that crushing volume of information and tease out the stuff in there that might interdict the next mass shooter?" asked Sweetow, adding that online content was "ultra-violent."
All three suspects in the Buffalo, New York, supermarket shooting in May, the in Uvalde, Texas last month, and now the Highland Park shooting Monday, were young men — ages 21 or under — with concerning online activity.
Nineteen states and the District of Columbiathat allow for the petition for the temporary removal of firearms from individuals who may be a danger to themselves or others. But Sweetow said those laws don't mean much if the concerning online activity is not reported to law enforcement.
"No matter how good the law is, and how much money the federal government pumps into it, if people will not pick up the phone or get on a keyboard and tell law enforcement about what they saw, the red flags are largely useless," he said.
While there is no hard and fast rule, Sweetow said a number of indicators can be concerning, such as loners who idealize violence and arm themselves.
"When you feel the hair stand up on the back of your neck, you really need to take that step and notify law enforcement," he said. "Notify someone in authority so that they can decide what needs to be done next."
These so-called lone wolf attacks can be among the hardest to disrupt, because suspects can very quickly cross the threshold from hateful speech online, which can have First Amendment protections, to violence.