(NewsNation) — Donald Trump is calling for unity Sunday morning as the FBI investigates Saturday’s assassination attempt on the former president at a rally in Butler, Pennsylvania. 

Trump, who campaign officials reported is “fine” following the shooting, took to Truth Social to address supporters. 

“In this moment, it is more important than ever that we stand United, and show our True Character as Americans, remaining Strong and Determined, and not allowing Evil to Win. I truly love our Country, and love you all, and look forward to speaking to our Great Nation this week from Wisconsin,” Trump posted. 

The Secret Service said one rally attendee was killed and two were critically injured in the shooting. Trump said they are in his prayers.

“Our love goes out to the other victims and their families. We pray for the recovery of those who were wounded, and hold in our hearts the memory of the citizen who was so horribly killed,” Trump said in the post.

A copy of an email from an RNC source shows Trump campaign staffers being asked to stay away from the DC and Palm Beach offices.

“In response to this violence, we encourage those of you who remain in DC and West Palm Beach to stay away from the office tomorrow, as we assess both locations,” the email from a top campaign official reads. “We are enhancing the armed security presence with 24/7 officers on-site.”

Early Sunday morning, the FBI identified the attempted assassin at Trump’s rally as 20-year-old Thomas Matthew Crooks. Secret Service shot and killed Crooks. 

The attack is the most serious attempt to assassinate a president or presidential candidate since Ronald Reagan was shot in 1981.

This is a breaking news story. Updates will be added here.