Unanswered questions remain after former President Trump assassination attempt

CHICAGO -- New details from a federal law enforcement bulletin describe former President Donald Trump political events, such as the one last Saturday, as "attractive targets." Investigators are trying to figure out what went wrong, and how to stop another attack from happening again.

Some Secret Service teams wear business suits; others wear tactical suits.

Some Secret Service officers stand watch. Others have rifles, and some rely on bomb sniffing dogs.

All are on a singular mission of protection. On Saturday, as ex-President Trump was making an illegal immigration point, there were gunshots.

Agents literally leapt into action, diving on Trump to shield him from gunfire, hustling the bloodied former president off-stage. Tactical officers with heavier weapons moved into place.

"Somehow an individual, who ultimately tried to assassinate the former president of the United States, was able to gain access to a rooftop 130 yards away," said John Cohen, former acting undersecretary for intelligence at the Department of Homeland Security.

Now an ABC News contributor, Cohen said witnesses apparently saw the shooter, 20-year-old Thomas Crooks, carrying a rifle and getting ready to attack.

"There are reports that attendees at the event reported to local police, the shooter making his way to that rooftop. Local authorities, as they were investigating, should have notified agents within the security bubble so they could assess whether removing the former president from that location was necessary to protect his safety," said Cohen.

A Homeland Security law enforcement memo states the shooter had recently "purchased 50 rounds of ammunition from a gun shop," received shipments of "packages marked hazardous material" and that two improvised explosive devices were found in Crooks' car and home.

"He appears to match the behavioral characteristics of mass shooters from the past," Cohen said. "If history is any indicator, what we will likely learn is that this was a disaffected, behaviorally challenged, angry person, who conducted this attack based on a blend of ideological, personal grievance and mental health-related factors."

The Homeland Security bulletin lays out a potent new threat during election cycles: "Domestic violent extremists who operate across ideologies" essentially see those who view the world differently as enemies who are viable targets for violence.

Authorities say they can't rule out copycat or retaliatory acts of violence in response to the assassination attempt. They also say lone wolf radicals make attacks challenging to prevent.