But a lawyer for Cherry said the people who claim to have heard Cherry's boasts are "inherently unreliable."
CBS News Correspondent Mark Strassmann reports the lawyer, Mickey Johnson, also said unreliable informants had misled the original detectives.
"This was never an investigation," he claimed. "It was a target. Let's build this case around Bobby Cherry."
Four decades later, the parents of the four murdered girls still want justice.
In fact, the mother of one of the victims was the first to take the stand, says CBS News Reporter Donna Francavilla.
Seated in a wheelchair, Alpha Robertson said she was at home getting ready for church when she heard what sounded like "something shaking the world all over."
Her daughter, 14-year-old Carole Robertson, and three other girls — Addie Mae Collins, Cynthia Wesley, both 14, and Denise McNair, 11 — died when a bomb went off outside Sixteenth Street Baptist Church on Sept. 15, 1963.
Denise's parents also sat in the front row as the trial opened.
Assistant U.S. Attorney Robert Posey opened Cherry's murder trial with black-and-white photos of the Sixteenth Street Baptist Church and children marching during a period of protests against segregation, taking the jury back nearly four decades when the church was a rallying site for demonstrators.
He said Cherry, 71, told authorities shortly after the bombing that "the only reason he didn't bomb the church is that someone beat him to it." Yet at the same time, Posey said, Cherry was boasting to family members that he was responsible.
"He has worn this crime on his chest like a badge of honor...a Klan medal," Posey said.
Later, Johnson told jurors they would not be able to trust the witnesses who claim they heard Cherry admit to the bombing. "He did not say this," said Johnson.
The jury, seated Monday, is made up of nine white women, three white men and four black men. Which four will be alternates rather than members of the 12-person jury will not be disclosed until later.
Johnson said black women were the largest group of potential jurors he removed from the panel. But Johnson said jury selection was based on more than race.
"I don't think the breakdown on racial lines will be as important as people think it will be," he said after the judge lifted a gag order imposed during jury selection.
Circuit Judge James Garrett had ruled that Cherry was not mentally competent for trial, but he changed his mind in January after experts concluded Cherry was faking mental illness.
The retired trucker, who lives in Mabank, Texas, was part of a group of Klansmen questioned by the FBI within days of the blast. He had always denied involvement.
However, he was indicted along with another ex-Klansman, Thomas Blanton Jr., on murder charges in May 2000.
Blanton was found guilty last year and is serving a life sentence. Another Klansman, Robert Chambliss, was convicted in the bombing in 1977 and died in prison. A fourth suspect, Herman Cash, died without being charged.
Cherry could get life in prison if convicted.