Civil rights charges against Zimmerman would be difficult, Justice officials say The weakness of the evidence compounds the political problems facing President Obama and Holder, who are under mounting pressure from many liberal and African American groups to bring a federal case against Zimmerman after a Florida jury acquitted him Saturday of second-degree murder and manslaughter. Obama has responded cautiously to the national uproar, making no public comments other than a carefully worded statement Sunday. Instead, Holder is acting as the administration's spokesman on the matter, saying in a speech Monday that Martin's killing was a "tragic, unnecessary shooting death." At a previously scheduled luncheon celebrating the Delta Sigma Theta sorority, a large audience of black women broke into applause when Holder said, "I share your concern." "We are determined to meet division and confusion with understanding and compassion - and also with truth," Holder said, adding: "We will never stop working to ensure that - in every case, in every circumstance and in every community - justice must be done." Justice Department lawyers are reviewing an investigation of Martin's shooting begun last year in conjunction with the FBI and state prosecutors in Florida, officials said. Prosecutors are combing through that evidence, as well as testimony from Zimmerman's state trial, to determine whether to file civil rights charges. "The Department of Justice couldn't bring this case unless they believe they could prove to a jury beyond a reasonable doubt that George Zimmerman killed Trayvon Martin because of his race," said Rachel Harmon, a law professor at the University of Virginia and a former prosecutor in the Justice Department's civil rights division. "It's not enough to show that Zimmerman followed Trayvon Martin because of his race," Harmon added. They would have to show that he attacked Martin for that reason. . . . Proving that motive is why it's hard to bring hate crime charges in general and why it is likely to be hard to bring them in this case. Privately, several Justice Department officials agreed that such charges would be difficult to bring for several reasons, including the difficulty in proving motive and the challenge posed by Zimmerman's acquittal in state court. Further complicating any federal case against Zimmerman are FBI interviews last year suggesting that racial bias was not a motivating factor in the killing. Florida officials released documents last year based on interviews with Zimmerman's ex- fiancee, neighbors, friends and co-workers, as well as with police investigator Christopher Serino in Sanford, Fla., the lead detective in the case. Serino said he thought Zimmerman followed Martin because of his attire, not his skin color. He told the FBI he thought Zimmerman had a "little hero complex" but did not believe he was a racist.