VIDEO New details emerge in Trump assassination attempt investigation

New details emerge in Trump assassination attempt investigation. The FBI said it was investigating the shooting as potential domestic terrorism.

After conducting more than 100 interviews, searching the suspect's home and vehicle, and cracking into his cellphone, FBI investigators were still searching on Tuesday for the motive that led a 20-year-old Pennsylvania man to attempt to assassinate a former president.

As fallout continued on the security failure that led to the attempt on former President Donald Trump's life, the Pennsylvania State Police said Tuesday afternoon that it was not responsible for securing the area where the gunman opened fire from a rooftop about 400 feet from the rally stage, hitting Trump's right ear, killing a bystander and wounding two others.

The state police said in a statement that it provided "all resources" to the Secret Service -- including 30 to 40 troopers to assist with securing the inside perimeter of the campaign rally venue in Butler Township, Pennsylvania -- but "was not responsible for securing the building or property" outside the security perimeter where the would-be assassin opened fire.

Feds warn of "retaliatory" acts

As the investigation continued, officials with the FBI, the agency spearheading the probe, and the Department of Homeland Security warned late Monday about the potential for "follow-on" or "retaliatory" acts.

"We cannot rule out the possibility that some DVEs or other actors may attempt follow-on or retaliatory acts of violence in response to this assassination attempt," the two agencies said in a joint bulletin, using an acronym for "Domestic Violent Extremism."

A cellphone used by the alleged assailant, Thomas Matthew Crooks, was sent to Quantico, Virginia, where FBI technicians gained access to its contents, officials said Monday.

Investigators did not immediately say if the analysis of the phone being conducted at the FBI laboratory yielded any clues on what drove the suspect to commit such a brazen attempt on Trump's life -- a key focus of the inquiry.

But investigators were expected to begin an analysis of the device's contents as they focused on search for a motive for the shooting that left one rallygoer dead and two others wounded, an FBI official said.

FBI officials also said a search of the suspect's vehicle and home in Bethel Park, Pennsylvania, about 53 miles south of where Saturday's campaign event occurred in Butler Township, has been completed. According to an FBI and Department of Homeland Security bulletin, two improvised explosive devices were found in Crooks' car and one in his home.

"Over the last several months, Crooks received several packages, including some marked as possibly containing hazardous material, according to a review of his shipping history," the bulletin said.

And the bulletin revealed that on the day of the shooting, Crooks purchased 50 rounds of ammunition prior to going to the rally in a gun shop in Bethel Park, Pennsylvania. It is unclear what shop the bulletin was referring to.

Suspect asked to borrow dad's AR-15-style rifle

ABC News learned that on the day of the shooting, Crooks told his father he wanted to go to a shooting range and asked if he could borrow his dad's AR15-style rifle, said a source familiar with the matter. Since the father had occasionally allowed Crooks to go target shooting with the semiautomatic firearm, he didn't think anything was suspicious when he let him use the rifle again on Saturday, the source said.

Just moments before the shooting, witnesses in the crowd attempted to point out the armed suspect to Secret Service agents. A Butler Township police officer climbed up on the roof and tried to engage the gunman who turned his weapon to the officer, causing him to retreat back down, Butler County Sheriff Michael T. Slupe told ABC News, adding that he was informed of the encounter by other law enforcement officials.

After being interrupted by the officer, the suspect proceeded to open fire in the direction of Trump, just as the presumptive GOP presidential nominee turned his head to look at a Jumbotron, a move Trump would later claim possibly saved his life.

The gunman was shot and killed by one of four Secret Service counter snipers positioned on rooftops around the rally, officials said.

The FBI is leading the investigation of the shooting.

FBI Director Christopher Wray called the assassination attempt on Trump "nothing short of an attack on democracy and our democratic process."

Homeland Security Secretary says shooter should have never gotten a clear shot

U.S. Secretary of Homeland Security Alejandro Mayorkas, who oversees the U.S. Secret Service, said Monday on ABC's "Good Morning America" that the 20-year-old gunman who attempted to assassinate Trump should never have been able to get a clear line of sight on the former president.

In an interview on "Good Morning America" with George Stephanopoulos, Mayorkas said the shooting should not have happened.

"George, a direct line of sight like that to the former president of the United States should not occur," Mayorkas said.

Investigators said Crooks used the rifle in the attempted assassination of Trump. The source also said that Crooks recently purchased ammunition online, as well as in-person.

Law enforcement has also been canvassing local gun shops in the Pittsburgh area for any information they can glean about Crooks' ammunition buying habits, both purchased in-store and shipped online. Those shops include Allegheny Arms & Gun Works in Bethel Park, roughly a mile from the Crooks' residence.

"As a responsible member of our community it is our prerogative to cooperate with law enforcement in every way," Allegheny Arms said in a statement to ABC News. "We are thankful that President Trump was not assassinated and our hearts and prayers go out to all victims of this horrible incident. Out of respect for the ongoing investigation and that of those affected we will not make any further statements."

FBI investigators have conducted nearly 100 interviews of law enforcement personnel, event attendees and other witnesses as they work to get to the bottom of what transpired at the rally, bureau officials said in a statement.

"The FBI has received hundreds of digital media tips which include photos and videos taken at the scene and we continue to review incoming tips," the FBI said in its statement.

Motive still unknown

The federal investigation into the attempted assassination of the former president is focused on the suspected gunman's motives for the crime and whether he was aided by others, authorities said.

In his address to the nation from the Oval Office Sunday night, President Joe Biden said it's unclear what prompted the suspect, identified as Crooks, to attempt such a brazen broad-daylight attempt on Trump's life.

"We don't know his opinions or affiliations. We don't know whether he had help or support or if he communicated with anyone else," Biden said. "Law enforcement professionals, as I speak, are investigating those questions."

Trump was about 10 minutes into his speech at an outdoor rally Saturday afternoon when shots rang out, followed by screams. The former president clutched his bleeding right ear and dove to the ground as Secret Service agents swarmed him, the FBI said in a statement late Sunday.

A bystander, 50-year-old Corey Comperatore, who worked as a local volunteer firefighter, was killed while trying to protect his family from the gunfire, according to Pennsylvania Gov. Josh Shapiro, who called Comperatore "a hero." Two other rallygoers shot in the incident were identified by the Pennsylvania State Police as 57-year-old David Dutch of New Kensington, Pennsylvania; and 74-year-old James Copenhaver of Moon Township, Pennsylvania.

Dutch and Copenhaver were listed in stable condition, officials said.

"The FBI has not identified a motive for the shooter's actions, but we are working to determine the sequence of events and the shooter's movements prior to the shooting, collecting and reviewing evidence, conducting interviews, and following up on all leads," the bureau said in the statement.

The building the suspect fired from was outside the security perimeter but was fewer than 400 feet from the podium where Trump was speaking, officials said.

Investigators have begun retracing the suspect's steps in an attempt to figure out how Crooks could have made it up onto the roof, which had been swept before the event, law enforcement sources told ABC News.

Mayorkas said an independent investigation announced by Biden on Sunday is already underway with a focus on the motive of the alleged gunman, identified as Thomas Matthew Crooks, who managed to get up on the roof of an agriculture building about 400 feet from the stage where Trump was speaking, investigators said.

"We have to learn everything about the assailant who, of course, the Secret Service neutralized," Mayorkas said. "We are going to really study the event independently and make recommendations to the Secret Service and to me so that we can assure the safety and security of our protectees which is one of our most vital missions in the Secret Service."

Mayorkas said it is important to have an "independent body" outside the Department of Homeland Security for "the integrity of the investigative process and the conclusions reached and the recommendations made have the full confidence of the government and the American public. And we intend to be transparent with respect to the findings and recommendations of the independent review."

Biden promised an independent investigation in the aftermath of the assassination attempt against Trump. Biden said the results of the independent investigation will be made public.

During the interview, Mayorkas rejected the "false" assertion that the Secret Service rebuffed requests for additional resources to Trump's security detail.

"We keep very close watch on a very dynamic threat environment. The president, the former president, are commonly consistently under threat," Mayorkas said. "We take every single threat seriously. We make security adjustments as are warranted. We had enhanced security for the former president, beginning in June. We had not received any requests for additional security measures that were rebuffed. That is false."

Biden also ordered the Secret Service to review security plans for the Republican National Convention in Milwaukee, which is scheduled to start on Monday. Mayorkas said he is confident in the security plan for the RNC.

Suspicious devices found in suspect's car and home

FBI investigators, led by the Pittsburgh Field Office, have searched Crook's home in Bethel Park, a suburb of Pittsburgh. They've also searched his vehicle, which was at the scene of the shooting in Butler, the FBI said.

"Suspicious devices found at both locations have been rendered safe by bomb technicians and are being evaluated at the FBI Laboratory," the FBI said.

The AR-15-style rifle allegedly used by Crooks had been legally purchased in 2013 by the suspect's father, law enforcement sources told ABC News. The family is cooperating with the investigation.

Secret Service director stresses getting to the bottom of what happened

In a statement released on Monday, U.S. Secret Service Director Kimberly Cheatle stressed that her agency is focused on learning how the gunman got a clear shot of Trump. She also expressed faith in the Secret Service's security plan for the RNC.

"The Secret Service is working with all involved Federal, state and local agencies to understand what happened, how it happened, and how we can prevent an incident like this from ever taking place again," Cheatle said. "We understand the importance of the independent review announced by President Biden yesterday and will participate fully."

Since the assassination attempt, Cheatle said she has been in constant contact with Secret Service personnel in Pennsylvania.

The FBI has taken over as the lead of the investigation.

"I have also been coordinating with the protective detail for former President Trump and have briefed President Biden on the details of the incident," Cheatle said.

Cheatle said the attempt on Trump's life "has understandably led to questions about potential updates or changes to the security" at the RNC. She said the agency has also implemented changes to Trump's security detail since the shooting "to ensure his continued protection for the convention and the remainder of the campaign."

"I am confident the security plan our Secret Service RNC coordinator and our partners have put in place, which we have reviewed and strengthened in the wake of Saturday's shooting," Cheatle said. "The security plans for National Special Security Events are designed to be flexible. As the conventions progress, and in accordance with the direction of the president, the Secret Service will continuously adapt our operations as necessary in order to ensure the highest level of safety and security for convention attendees, volunteers and the City of Milwaukee."