Santorum was critical of local prosecutors, who declined to arrest the shooter, George Zimmerman, a 28-year-old neighborhood watchman who claimed self-defense and invoked Florida's' "Stand Your Ground" law. The law gives people wide latitude to use deadly force instead of retreating during a fight and explains why Zimmerman has not been arrested.
The failure to prosecute is a "chilling example of the horrible decisions made by people in this process," Santorum said. "Stand your ground is not doing what this man did."
Gingrich also criticized the shooter, whom he described as "a guy who'd found a hobby that's very dangerous."
Romney, like Obama, called the incident "a tragedy," and called for "a thorough investigation that reassures the public that justice is carried out with impartiality and integrity."
While attention to Martin's death overshadowed presidential politics at times Friday, Santorum tried to keep the pressure on Romney ahead of the Louisiana contest. Santorum argued that he is the only Republican candidate who can offer voters a stark contrast with Obama.
After testing his marksmanship with a .45 caliber semiautomatic Colt pistol, he told reporters: "If you don't have a choice, then a lot of voters are going to vote for what they have. That's why we have to have a choice."
Romney heads into Louisiana with a commanding delegate lead in the race to 1,144, the number needed to clinch the GOP nomination.
Romney has earned 563 delegates so far, compared to 263 for Santorum, 135 for Gingrich and 50 for Ron Paul, according to an Associated Press tally.
Louisiana offers a total of 46 delegates, but only 20 will be up for grabs in Saturday's primary. An additional 23 will be selected at the party's state convention in June and three others go to the state's Republican National Committee members.
And while Santorum tried to recover from one misstep, a supporter in the audience added an off-message wrinkle. As he fired the pistol, a woman in the crowd shouted: "Pretend it's Obama."
Santorum was wearing protective ear muffs. He said later that he didn't hear the remark but denounced it as "absurd."
The Secret Service, which provides security for Santorum, was trying to identify the woman.
Associated Press writer Kasie Hunt from Louisiana contributed to this report.
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