Two weeks after former President Donald Trump survived an assassination attempt that left a bystander dead and two others wounded, and after days of Capitol Hill testimony, several burning questions remain unanswered but could make sense of the case, experts say.

Among those who appeared before Congress were Kimberly Cheatle, who was the director of the U.S. Secret Service (USSS) until resigning shortly after testifying, FBI Director Christopher Wray and Pennsylvania State Police Col. Christopher Paris.

After law enforcement spotted the would-be assassin, Thomas Matthew Crooks, and labeled him a person of interest, he managed to drop out of view and climb up onto a rooftop with a direct line of site of the former president. Then he opened fire, striking Trump in the ear with a bullet, according to the FBI, killing a man in the audience, and critically wounding two others.

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"That question is the key," said Paul Mauro, a retired NYPD inspector who has been in the command center for other VIP events. "That’s where the investigation is going to ultimately sum up."

The fact that law enforcement had identified Crooks as a suspicious individual long before Trump took the stage is indicative of a communication failure, he said.

Authorities were actively looking for Crooks when Trump walked out to begin his remarks, and if the gunman hadn't been thrown off kilter by a local cop who peeked over the edge of the roof in search of the suspect, he could have been killed by the first shot, Mauro said.

"If it got to that level, hold Trump for 20 minutes until you find him," he added.

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After prompting criticism for saying there were no agents stationed on the rooftop because it was "sloped," Cheatle told lawmakers last week that the Secret Service "prefers sterile rooftops" – meaning secured in a way that no one would be able to get on top.

But the would-be assassin was able to climb up despite being identified as a suspicious person well before the shooting.

"That's the elephant in the room," said Michael Verden, a former Secret Service agent and the founder of the Lake Forest Group, a security firm. "What needs to be determined is why there was not a presence on that rooftop."

Paris, Pennsylvania's state police commissioner, said that during a walkthrough before the rally, the Secret Service told him that a Butler County tactical team had been assigned to cover the rooftop. 

There are multiple ongoing investigations related to the event, some focused on the security failures and others on Crooks himself. This question will be consequential for people involved in planning and securing the event.

"For the USSSS investigation, it’s important to know, so the appropriate individual is held accountable," said Nicole Parker, a former FBI agent. "The idea that law enforcement had identified a potential threat, then why didn’t they cover every potential vulnerability. It should have been locked down anyway, but especially when they learned of a suspicious or threatening individual."

Little is known about Crooks' motive. The FBI said last week that it is one of the main focal points in their investigation.

"What was the event or series of events that sparked his decision to go on the pathway to violence?" asked Parker, who has experience conducting mass shooting investigations after the Parkland massacre in Florida. 

She said agents are trying to identify the "precipitating stressor" that changed Crooks from a college student living with his parents into a would-be assassin.

Authorities have said Crooks had no criminal history and no documented mental illnesses. The murder weapon was purchased legally by his father in 2013.

Last week's congressional testimony revealed little about potential radicalization or ideology, but a former classmate told Fox News Digital that Crooks "did not like politicians."

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Who, if anyone, was Crooks communicating with on his encrypted apps?

Crooks had an unusually empty online presence, but the FBI is analyzing his devices.

FBI Director Christopher Wray testified last week that investigators have already found disturbing Google searches – including the phrase, "how far away was Oswald from Kennedy?" But he also mentioned that Crooks was using encrypted messaging apps.

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While authorities say they believe he acted alone, there could be clues in his communications.

Virtually all present-day criminals leave behind some kind of evidence on their devices, according to Ashton Packe, a former Las Vegas detective who served on the FBI's Joint Terrorism Task Force. 

"I find it super interesting that there's encryption involved," Verden told Fox News Digital. 

That can slow efforts to dig into his digital footprint, he said.

"I'm not a conspiracy theorist, but we've got to confirm that he acted alone, and he wasn't working with someone else," he added. "And that's a really big part of this investigation…this guy's background and what he did leading up to that day." 

Crooks fatally struck 50-year-old Corey Comperatore and critically wounded David Dutch, 57, and James Copenhaver, 74.

Fox News' Sarah Rumpf-Whitten contributed to this report.