It was only "a matter of seconds" between the time that the gunman, Thomas Matthew Crooks, had been positively identified as a threat with a gun -- as opposed to just a suspicious person -- and when he first opened fire, Pennsylvania State Police Col. Christopher Paris told the House Homeland Security Committee on Tuesday.

Paris said Crooks was identified as a "suspicious" person because he was staying outside the venue perimeter and not entering the venue, and he was "milling around" the area in front of the AGR building, where he later fired from. But at that point, Crooks was just one of four people "identified as suspicious" at the Trump rally, Paris said.

When Butler County ESU members saw Crooks had a rangefinder, "the suspicion was heightened," and they sent that information and a photo of him to state police inside the command post, who "immediately" relayed it "verbally" to Secret Service, Paris said. State police also sent the photo of Crooks to a Secret Service number, he said.

That was about 20 to 30 minutes before Crooks opened fire, Paris said.

"But at that point. just to be clear, he was determined to be suspicious," Paris emphasized. "There was no information that he possessed a weapon."

The ESU members at the AGR building and at least two municipal officers in the area then "were actively looking for Crooks," Paris said.

"They circled the building, but then it became apparent at some point that he was on the roof," Paris said.

So one of the municipal officers "boosted the other one up, hanging from the ledge of the roof," he said. "By the time that officer was boosted up on top of the roof, Crooks was on it, almost in that final position that you saw, he had the AR out, and he pointed it at the municipal officer who was suspended from the roof. [The officer] was not in a position -- feet dangling at that point --- to draw a weapon. ... He let go, and he fell back down."

Meanwhile, other law enforcement was scrambling on the ground to find the suspicious person but couldn't see him on the roof, Paris said.

It was "a matter of seconds" between the officer falling and Crooks opening fire, Paris said.

-ABC News’ Luke Barr and Mike Levine

House Speaker Mike Johnson recounted to ABC News on Tuesday what he told then-Secret Service Director Kim Cheatle when they met one-on-one on Monday.

"I made it clear, as I always do to anyone in these positions, 'This is not personal, ma'am,'" Johnson recalled. "'But you're No. 1 responsibility is performance -- and you did not perform. In fact, it was an utter failure, and we almost lost a former president because of it. And accountability begins at the top and you need to take responsibility.'"

Pennsylvania Gov. Josh Shapiro said Tuesday that Cheatle’s resignation was "the right thing to do."

"I mean, there were obviously incredible failures at the farm show in Butler. There are many investigations going on, and those investigations need to unearth what those failures were, so that any protectee is ensured the absolute top level of protection going forward. That was a failure in Butler. Thank God the former president was not assassinated," Shapiro said.

-ABC News’ Jay O’Brien and Will McDuffie

During a walk-through on July 11, the Pennsylvania State Police was told by the Secret Service that the Butler County Emergency Services Unit was responsible for security of the AGR building, where the shooter fired from during the July 13 rally, according to state police Col. Christopher Paris.

Paris told the House Homeland Security Committee on Tuesday that two Butler County ESU members were on the second floor of the building and positioned in the window.

Crooks was "initially spotted" from a second-story window and identified as a suspicious person, which prompted then ESU officers to leave their post and go search for the suspicious person, along with other officers, Paris said.

Paris said the gunman, Thomas Matthew Crooks, never made it to the safety perimeter itself.

"He was identified by those members as suspicious, in part because of that, and then at some point, he produced the rangefinder, which heightened that suspicion," Paris testified.

Police realized "very early on" that Crooks had a detonation device on his person, Paris said. It was a "serious tactical consideration" that Crooks could have blown up his vehicle and escaped from the scene, Paris said, but noted that because he was dead, there was no way to know.

-ABC News’ Luke Barr and Mike Levine

Secretary of Homeland Security Alejandro Mayorkas said he’s appointed Secret Service Deputy Director Ron Rowe to serve as acting director of the agency.

"A 24-year veteran of the Secret Service, he previously served as the agency’s Assistant Director for the Office of Intergovernmental and Legislative Affairs, Deputy Assistant Director for the Office of Protective Operations, and in other leadership positions," Mayorkas said in a statement. "I appreciate his willingness to lead the Secret Service at this incredibly challenging moment, as the agency works to get to the bottom of exactly what happened on July 13 and cooperate with ongoing investigations and Congressional oversight."