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."

The Pennsylvania State Police has interviewed 100 people and gathered 1,000 pieces of evidence in the investigation into the homicide of Corey Comperatore and the two others injured in the shooting, Pennsylvania State Police Col. Christopher Paris told the House Homeland Security Committee on Tuesday. The state police is also investigating the U.S. Secret Service counter sniper who shot the gunman as an officer-involved shooting.

Paris said the state police had two main responsibilities: to assist the Secret Service with the motorcade and staff security posts within the secure perimeter.

The state police had 32 troopers staffed for the July 13 rally, he said.

Paris said the Secret Service notified the state police about the rally on July 5.

An initial planning meeting with law enforcement agencies was held on July 8. State police participated in a walk-through at the Butler Farm Show Grounds on July 11.

The state police said it provided all resources requested by the Secret Service.

-ABC News’ Luke Barr and Mike Levine

House Oversight Committee Chairman James Comer, R-Tenn., said Secret Service Director Kim Cheatle’s resignation is "a step toward accountability," but a "full review" is still needed "of how these security failures happened so that we can prevent them going forward."

"We will continue our oversight of the Secret Service in support of the House Task Force to deliver transparency, accountability, and solutions to ensure this never happens again," Comer said.

President Joe Biden said in a statement, "Jill and I are grateful to Director Kim Cheatle for her decades of public service. She has selflessly dedicated and risked her life to protect our nation throughout her career in the United States Secret Service."

"The independent review to get to the bottom of what happened on July 13 continues, and I look forward to assessing its conclusions," he said. "We all know what happened that day can never happen again."

Biden said he will appoint a new director "soon."

6:07

Embattled Secret Service Director Kim Cheatle resigns

On Monday, Cheatle told Congress that the attempted assassination against former President Donald Trump was the "most significant operational failure at the Secret Service in decades."

House Speaker Mike Johnson called the resignation "overdue."

"We need the Secret Service to be acting -- to be performing -- at the top of their game, and we'll ensure that that happens. I hope that the interim director or whomever is appointed to replace Director Cheatle will be very sober minded about the responsibility and take into account the very dangerous times in which we live," Johnson said.

-ABC News’ John Parkinson