Something seemed off from the moment Beaver County SWAT sniper Gregory Nicol spotted a man skulking around the outskirts of the site where former President Donald Trump was about to take the stage on July 13.
From his second-floor post inside the AGR complex at the fairgrounds in Butler, Pennsylvania, Nicol noticed the young man in a gray T-shirt, lurking.
"He was looking up and down the building ... It just seemed out of place," Nicol, assistant leader of the Beaver County SWAT team, told ABC News in an interview airing Monday at 7 a.m. ET on "Good Morning America." "It just didn't seem right."
Nicol noticed an unattended bike and backpack. And he saw the man looking up and around, then pulling a rangefinder from his pocket. There was no apparent reason to have a distance-gauging device at a political rally featuring the man who, in a few days, would accept his party's presidential nomination. The sharpshooter snapped pictures of the suspicious-looking man and the bike, then flagged it to fellow snipers from his team assigned to the event and called it into the command group.
Nicol would be the first officer to issue a warning about 20-year-old Thomas Matthew Crooks. Within an hour, Crooks would open fire from the roof of that very building, less than 200 yards from the rally's stage, wounding Trump on live TV, killing one person in the crowd, and critically injuring two more.
The sniper and his fellow Beaver County SWAT officers were assigned to Trump's Butler campaign rally, and tasked with supporting the Secret Service and other law enforcement in the mission to keep the event and Secret Service protectee, safe.
They have not spoken publicly until now.
In their first public comments since the assassination attempt, the Beaver County SWAT team and their supervisors spoke with ABC News Senior Investigative Correspondent Aaron Katersky, marking the first time any of the key law enforcement personnel who were on site July 13 have offered firsthand accounts of what occurred.
The violent episode has already led to the resignation of Secret Service Director Kimberly Cheatle. And, in the wake of the assassination attempt, a series of law enforcement, internal, and congressional probes have been announced -- with communications and coordination a key focus of investigators' attention.
"This one is something that we'll always carry with us," assistant Beaver County SWAT leader Mike Priolo told ABC News.
Long before Crooks would fire his AR-style rifle that Saturday evening, Crooks' presence wasn't the only thing that didn't seem quite right to the local SWAT team.
Team members said that the day of the rally, they had no contact with the agents on Trump's Secret Service detail.
"We were supposed to get a face-to-face briefing with the Secret Service members whenever they arrived, and that never happened," said Jason Woods, team leader for Beaver County's Emergency Services Unit and SWAT sniper section.
"So I think that was probably a pivotal point, where I started thinking things were wrong because it never happened. We had no communication," Woods said. "Not until after the shooting."
By then, he said, "it was too late."
The Secret Service, whose on-site team was supplemented as usual by local, county and state law-enforcement agencies, was ultimately responsible for security at the event, but none of the concerns apparently reached members of Trump's detail. The Washington Post reported over the weekend that Secret Service agents have complained they were not made aware of the warnings.
Secret Service spokesperson Anthony Guglielmi declined to respond directly to the comments Woods and his colleagues made to ABC News. He said the agency "is committed to better understanding what happened before, during, and after the assassination attempt of former President Trump to ensure that never happens again. That includes complete cooperation with Congress, the FBI and other relevant investigations."
MORE: Local SWAT snipers saw Trump rally gunman nearly 2 hours before assassination attempt, text messages showTo the men and woman of Beaver County SWAT, what happened is clear: There was a lack of planning and communication that caused a catastrophic failure in the protection of Donald Trump. They said they saw the problem coming, and they tried to alert the people in charge and sound the alarm.
With the presidential campaign in full gear and Trump now saying he wants to return for another rally outside Pittsburgh, it is critical to know what went wrong at the last one -- so it doesn't happen again.
"I have to imagine that they're going to make some very serious adjustments -- namely, probably, hold it inside where you have a lot more control over who's coming in," said Beaver County District Attorney Nate Bible, who oversees the county SWAT unit. "If we're asked for assistance, we will provide it."
By mid-morning on July 13, the Beaver County team of snipers and spotters was in position -- hours before Trump was set to take the stage that evening at the sprawling grounds that's studded by a complex of warehouses.
Once they were positioned at the security perimeter -- outside the metal detectors -- Woods said he immediately wondered whether they had been put in the most effective spot.
"I think the better location would have been inside looking out, and that's actually where the Secret Service snipers end up getting placed," Woods said. "For us to effectively do our job, I don't know if that was the best location."
But it was "an away game," Woods said, meaning his team was not in charge. So they deferred to the Secret Service agents whose job it was to determine the security plan and keep Trump safe.
"I knew the Secret Service knew where we were supposed to be, and that's where we were placed," Woods said.
"Our instructions, marching orders were given to us from Butler County EMS unit, their command. With, historically speaking, approval from the Secret Service," Priolo said.
MORE: Failure of communication: Local SWAT team details account of Trump rally assassination attemptThis was not the team's first time participating in a Secret Service operation.
"We as a team would assume that that would be a robust type thing, that they would have constant communication. And it very well might have been -- we're just not aware of it," said Beaver County Chief Detective Patrick Young, the commander of emergency services.
The event's atmosphere, Young said, also meant a dynamic environment: Officers had to rapidly gauge whether rallygoers' bulging back pockets held merely bottled water or booze -- commonplace at a festive gathering under the blazing summer sun of Western Pennsylvania -- or was a sign of something more sinister.
"Our first indication that there was going to be something different about this was the lack of patrol that we'd seen in the area," Priolo said of the plans.
The effect of that, he said, was that the SWAT officers would have to personally handle any urgent patrol-level incident that should arise.
"The best analogy I've heard is -- we're a scalpel, when you're asking us to be used as a hammer," Priolo said. "That's kind of what we figured out throughout the day."
When Nicol observed Crooks' suspicious presence and called it in to local command via radio, he said he expected action to be taken -- like a uniformed officer would "check it out," according to text messages between snipers on the ground, which were obtained by ABC News.
"The first thing I did, I sent those pictures out, we had a text group between the local snipers that were on the scene. I sent those pictures out to that group and advised them of what I noticed and what I'd seen," Nicol said. 'There was a text back that said, 'Call it into command.' I then called into our to the command via radio. And they acknowledged."
"I assumed that there would be somebody coming out to -- you know, to speak with this individual or, you know, find out what's going on," he added.
Nicol moved through the building trying to shadow Crooks, who was outside, and keep eyes on him. But Nicole lost sight of Crooks as Nicol made his way down to the building's first level.
By that time, Trump had taken the stage, Nicol said.
Then, as the former president began speaking, Nicole noticed rallygoers looking away from the podium, up toward the roof of the AGR building. Some were shouting that there was someone up there.
Nicol said he was almost relieved, thinking to himself, "Oh, they must have found this guy we were looking for out there, and everybody's watching the police deal with him."
He would soon discover that wasn't the case.
"That's when I heard the gunshots," Nicol said. Crooks had opened fire on the campaign rally.
SWAT medic Michel Vasiladiotis-Nicol responded with Beaver County SWAT Det. Rich Gianvito, along with other local personnel from Butler County and the surrounding areas.
They squeezed through the fence perimeter and headed toward the building where the shots had come from.
"We then ascended that ladder to then meet up with -- what -- we weren't sure again if it was a mass casualty or what we were walking into," said Vasiladiotis-Nicol, who is sniper Gregory Nicol's wife.
MORE: FBI says bullet struck Trump's ear during assassination attempt"We're prepared for anything at that point," Gianvito said, including a possible firefight because the team had no idea if the rooftop shooter was dead or alive, or if there could be an accomplice still unaccounted for.
On the roof, they found Crooks motionless and face down -- images captured on Gianvito's helmet camera. Crooks' wrists had been quickly bound with white plastic ties, in case he was still alive. A long trail of blood flowed down the sloped roof.
Vasiladiotis-Nicol put her gloved fingers to the shooter's neck. "He had absolutely no pulse," she recalled.
In the seconds after the shooting, Trump was rushed to a local hospital, where doctors treated a wound to his ear. Later that night he flew back to his golf club in New Jersey. The first photos of him after the shooting -- blood down his face, fist raised over the heads of the Secret Service agents rushing him away -- have already become iconic images.
What remains are looming questions and an impatient Congress. How could this happen? Could the shooting have been prevented? Was it a failure of planning, coordination, communications -- or all of the above?
"I think with some better planning perhaps, it could have been stopped," said Bible, the Beaver County DA. "You're protecting one of probably the more high-profile political candidates in history. So, how was a 20-year-old able to fire off several shots at him?"
Watch ABC News' exclusive first interview with the local SWAT team on the ground during Trump's assassination attempt, airs in its entirety on "Good Morning America" on Monday, July 29, at 7 a.m. ET.