Federal authorities are investigating a shooting that occurred at a rally in Butler, Pennsylvania, on Saturday as an assassination attempt on former President Donald J. Trump. The incident resulted in the death of one rally attendee and left two other spectators critically injured. The suspected gunman was killed by responding officers.

The Secret Service confirmed that Mr. Trump was escorted off the stage after sustaining an injury. A bullet had pierced the upper part of his right ear, causing him to bleed. He was taken to a hospital for treatment and later posted on social media to confirm his condition.

Special Agent Kevin Rojek of the FBI’s Pittsburgh field office stated at a news conference that investigators are “working tirelessly” to determine the motive behind the shooting. As of now, the authorities have not identified the gunman or speculated on his motives.

President Joe Biden condemned the violence in a televised statement, calling it “sick.” The White House later confirmed that President Biden had spoken with Mr. Trump.

The shooting occurred just minutes into Mr. Trump’s speech while he was showing supporters a chart about border crossings.

Law enforcement officials recovered an AR-15-type semiautomatic rifle from the deceased gunman. Rally attendees undergo security screenings, and their belongings are typically searched for weapons. President Biden expressed gratitude for Mr. Trump’s swift evacuation and denounced the violence. His campaign announced a pause on “all outbound communications” and efforts to “pull down our television ads as quickly as possible.”

Both Republicans and Democrats condemned the apparent act of political violence. The incident has prompted widespread calls for increased security at political events. Despite the shooting, the Republican National Convention, where Mr. Trump will be formally nominated as the party’s presidential candidate, will proceed as planned in Milwaukee starting on Monday, according to a statement from the Trump campaign and R.N.C. officials.

The investigation continues as authorities work to uncover more details about the attack and ensure the safety of future political events.