Donald Trump's plane landed at Milwaukee International Airport Sunday evening ahead of the Republican National Convention.

Trump’s highly anticipated appearance at the RNC on Monday comes just two days after the assassination attempt of him at Saturday's campaign rally in Pennsylvania.

Authorities outside the security perimeter of Donald Trump's campaign event Saturday were alerted there was a man acting suspiciously before the shooting.

Local and state police, responsible for security outside that perimeter, began to search for a potential suspect, multiple sources briefed on the probe tell ABC News.

Within minutes, law enforcement spotted the suspected gunman on the roof, sources said.

It is unclear as of now, according to sources, whether this information was passed along to the U.S. Secret Service, which is responsible and focused on the security of the former president and the crowed inside the security zone.

A police officer saw the suspect on a rooftop and attempted to make it to the roof to engage the shooter, police sources said.

That officer was threatened when the gunman pointed his rifle at the cop, according to sources, who add that effort did not succeed as, moments later, shots rang out, targeting Trump.

-ABC News' Josh Margolin

President Joe Biden will deliver "a forceful and needed" address to the nation Sunday evening following the assassination attempt of former President Donald Trump on Saturday, a campaign official told ABC News.

Biden will address the "horrifying attack" and urge "every American to come together to not just condemn, but put to an end political violence in this country once and for all," the official said.

In light of Saturday’s shooting, Biden’s campaign said they were "pausing all outbound communications and working to pull down our television ads as quickly as possible."

The campaign official said next week, "The DNC and the campaign will continue drawing the contrast between our positive vision for the future and Trump and Republicans’ backwards-looking agenda over the course of the week."

In the ongoing investigation into the attempted assassination of Donald Trump Saturday, FBI officials said "suspicious devices" were found in the suspected shooter, Thomas Matthew Crooks' car and home.

Pittsburgh Special Agent in Charge Kevin Rojek said there was a suspicious device found in the suspect's car and agents were trying to determine what it is.

Another was found in his home, Rojek said, and both were described as rudimentary.

Crooks had no known history of mental illness, per the FBI. His family is cooperating with investigators.