• The motive of the 20-year-old gunman who tried to assassinate former President Donald Trump remained unknown three days after the shooting at a campaign rally in western Pennsylvania.
• More than a dozen guns were found by authorities in a search of the home of the shooter, Thomas Crooks.
• The U.S. Secret Service continues to face questions about how Crooks was able to crawl up to a roof overlooking the site of Trump's rally.

The motive of the 20-year-old gunman who tried to assassinate former President Donald Trump remained unknown three days after the shooting at a campaign rally in western Pennsylvania.

Information analyzed from a phone belonging to the gunman, Thomas Crooks, has not helped the FBI determine what led him to fire a rifle multiple times at Trump and other rally attendees Saturday afternoon before he was killed by a Secret Service sniper, a senior law enforcement official told NBC News.

Authorities found more than a dozen guns were in a search of the Crooks family home.

Two suspected improvised explosive devices were found in Crooks' car near the rally, while a third suspected IED was found at his home.

A joint intelligence bulletin issued by the FBI and Department of Homeland Security warned that extremists may conduct attacks in retaliation for the assassination attempt, a source told NBC News.

"The FBI and DHS remain concerned about the potential for follow-on or retaliatory acts of violence following this attack, particularly given that individuals in some online communities have threatened, encouraged, or referenced acts of violence in response to the attempted assassination," the bulletin said, according to the source.

That same bulletin confirmed that Crooks purchased 50 rounds of ammunition at a store in his hometown of Bethel Park, Pennsylvania, on Saturday before he drove to the rally.

The FBI spoke to more than 100 witnesses, family members of Crooks and friends, rally attendees and other law enforcement officials in an effort to map out the gunman's steps leading up to the shooting and determine what motivated him to do it.

Meanwhile Tuesday, the Secret Service, which is responsible for protecting Trump, continued to face strong criticism for not taking precautions that might have prevented Crooks from crawling onto a roof overlooking the rally site with his rifle and firing shots that injured the former president, killed one man and wounded two others.

The shooting occurred two days before the Republican National Convention kicked off in Milwaukee and formally nominated Trump as the GOP's presidential candidate for the November election.

Local police alerted Trump's Secret Service detail before the shooting that they were looking for a suspicious person in the area of the rally, a U.S. official told NBC News. That person was later identified as Crooks, who had been seen pacing and acting oddly near magnetometers at the event.

Secret Service Director Kimberly Cheatle told ABC News in an interview that the shooting "was unacceptable ... and it's something that shouldn't happen again."

"The buck stops with me," Cheatle said. But she also said she would not resign over the incident.

In the interview, Cheatle said that local police in Butler Township, Pennsylvania, were responsible for securing the building where Crooks got on the roof after being seen by witnesses who shouted about his presence there

"In this particular instance, we did share support for that particular site and that the Secret Service was responsible for the inner perimeter," Cheatle said. "And then we sought assistance from our local counterparts for the outer perimeter."

"There was local police in that building — there was local police in the area that were responsible for the outer perimeter of the building," she said.

Cheatle told ABC that the building had a sloped roof, which made it challenging for authorities to secure it, which is what led to the decision to have police inside the building.

The Secret Service in a statement early Tuesday that was released after Cheatle's comments pushed back at the idea the agency was blaming local police for failing to stop the gunman.

"Our agency relies on the support of courageous police officers and local partners. We are deeply grateful for their unwavering commitment and bravery," the Secret Service said. "Any suggestion otherwise is simply not true."

"We are deeply grateful to the officers who ran towards danger to locate the gunman and to all our local partners for their unwavering commitment," the agency said. "Any news suggesting the Secret Service is blaming local law enforcement for Saturday's incident is simply not true."