Live updates: Biden calls for unity as FBI investigates lone shooter at Trump rally

President Biden called for unity in a primetime address on Sunday, urging Americans to resolve their differences "at the ballot box, not with bullets," just over 24 hours after a gunman opened fire on a Trump rally Saturday.

Law enforcement officials told reporters on Sunday that they are investigating a lone shooter, adding that the probe is being considered an act of domestic terror.

Biden has also ordered a full review around the security for Trump's Pennsylvania rally, and to assess the security in place for the Republican National Convention (RNC) this week in Milwaukee.

Trump arrived in Wisconsin Sunday evening for the RNC as planned. He'd thanked the nation on Sunday morning for its concern in the wake of an attempt on his life the evening before.

Amid bipartisan condemnation of political violence, there are also growing questions about how the suspected shooter — Thomas Matthew Crooks, 20, of Bethel Park, Pa. — was able to get so close to the former president, firing a bullet that grazed his ear.

Corey Comperatore, 50, a volunteer firefighter, was also killed in the shooting, reportedly after throwing himself on top of his family to protect them. Two others were critically injured, officials said.

Follow along for live updates here.