He urged readers to “turn toward each other with all the compassion we gave Mollie.”

“Let’s listen, not shout. Let’s build bridges, not walls,” he wrote. “We have the opportunity now to take heed of the lessons that Mollie, John McCain and Aretha Franklin taught — humanity, fairness and courage.”

Ms. Tibbetts, 20, was last seen while on a run in her hometown, Brooklyn, Iowa, on July 18. Television crews swarmed the town as the authorities searched for her and conducted hundreds of interviews. Surveillance video led them to Mr. Rivera, who they said later led them to her body.

It did not take long for politicians to seize on her killing and use it to promote their own platforms. On the day the discovery of her body was announced, President Trump, in a rally in West Virginia, used her death as a talking point about the state of immigration laws. He has repeatedly linked crime to illegal immigration, despite studies that show otherwise.

Candace Owens, a conservative commentator, tweeted about Ms. Tibbetts, saying that Democrats would not be outraged about her death “because they need open borders for votes.”

Ms. Owens also took note of the outrage from liberals and Democrats about children of undocumented immigrants being separated at the border of the United States and Mexico, and asked, “What will they do for Mollie Tibbetts?”

Sam Lucas, a distant relative of Ms. Tibbetts, tweeted back at her, saying her family is not so “small-minded that we generalize a whole population based on some bad individuals.” She told The Washington Post, “I wanted to protect my family from this extra grief of politics.”

Billie Jo Calderwood, Ms. Tibbetts’s aunt, told CNN that she did not want her niece’s memory to get lost in politics.