PHOTOS: Shots fired at Trump rally in Pennsylvania in possible assassination attempt

Photos show former Pres. Donald Trump swarmed by Secret Service agents and rushed off stage after shots were fired during a rally in Butler, Pennsylvania on Saturday.

Officials briefed on the investigation told ABC News that the shooting -- a chilling development on the eve of the Republican National Convention -- was being investigated as a possible assassination attempt and that live rounds were apparently fired from outside the security perimeter.

The Secret Service, in a statement, said that a shooter "fired multiple shots toward the stage from an elevated position outside of the rally venue" after which agents "neutralized the shooter, who is now deceased." They went on to say that one spectator was killed and two others critically injured.

A spokesperson for Trump said in a statement, "President Trump thanks law enforcement and first responders for their quick action during this heinous act. He is fine and is being checked out at a local medical facility."

Take a look at the latest updates on the deadly shooting here.