Moments after she heard gunshots, Chasitty Logsdon, a 39-year-old nurse from Louisville, Ky., said she ran over to a man on the ground nearby, blood pooling around him. A woman beside the man told Ms. Logsdon that he had been shot in the head, so Ms. Logsdon took his pulse and started doing compressions. Once medics arrived, Ms. Logsdon said, they took over. She said she knew she had to do what she could to try to save the man.

“If that was my someone,” Ms. Logsdon said she remembered thinking, “I would want the same thing.”

The governors of Kansas and Missouri were both at the rally. Gov. Laura Kelly of Kansas had to be evacuated, and she posted on social media that she had been taken “out of harm’s way.” Gov. Mike Parson of Missouri and his wife were both “safe and secure,” the governor’s office said on social media.

Quinton Lucas, the mayor of Kansas City, Mo., said he was at the parade with his wife and mother, and was in Union Station when he heard gunfire.

“When you have people who decide to bring guns to events, when you have people who are deciding to try to mar events — celebratory ones, like this one — all of us start to become members of this club that none of us want to be a part of,” Mr. Lucas said.

Kansas City players posted messages of support on social media. Patrick Mahomes, the quarterback who led his team to victory over San Francisco on Sunday, said he was “praying for Kansas City.” The tight end Travis Kelce wrote that he was “heartbroken,” adding, “KC, you mean the world to me.”

Gun violence has been falling in some parts of the country, but Kansas City, Mo., saw a record number of homicides in 2023. There were 183 murders last year, more than the previous record of 179 in 2020.

Reporting was contributed by Traci Angel, Gaya Gupta, Robert Gebeloff and Jesus Jiménez. Susan C. Beachy and Sheelagh McNeill contributed research.