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 wouldn'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 are going to have to show that he attacked Martin for that reason. . . . That's why it's hard to bring hate crimes in general and 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.
The jury in Florida acquitted Zimmerman after his attorneys used a self-defense argument.
Zimmerman, 29, a neighborhood watchman, saw Martin walking through his gated community on a rainy night in February 2012 and followed him. Zimmerman said he shot Martin after the teenager punched him, knocked him down and started pounding his head into a concrete walkway. Prosecutors said Zimmerman racially profiled Martin and initiated the confrontation that resulted in the teenager's death.
The verdict brought protests from civil rights organizations and demands for federal action. Among those calling on the Justice Department to become involved were three black Democratic congressmen from New York: Charles Rangel, Hakeem Jeffries and Gregory W. Meeks.
The NAACP started a petition late Saturday to urge the department to file civil rights charges against Zimmerman. The petition received such a massive response that it crashed the organization's website.
Obama, the nation's first African-American president, has often struggled to find his footing in confronting sensitive issues of race. And Holder, the nation's first black attorney general, increasingly has become Obama's voice on controversial racial issues.
Holder is a liaison to black advocacy organizations and pushes an aggressive civil rights agenda, at times providing the White House cover when Obama chooses not to get involved. On Tuesday, Holder will address the NAACP's annual convention in Orlando, Fla., not far from Sanford, where the trial was held.
Joshua DuBois, a former White House adviser to Obama on spiritual issues, said the president tends to engage on sensitive matters such as the Martin case only if he thinks his leadership can help the situation.
"He runs these things through a couple filters - one, will it on balance bring more healing than division, and two, will it accomplish a specific policy objective," DuBois said. "The activist community, their job is to want more, to be the voice of the voiceless. But the role of the president is to bring the country together."
In March, Obama inserted himself into the controversy surrounding Martin's death by making a personal appeal to the teenager's parents and parents everywhere. "If I had a son, he'd look like Trayvon," the president said from the Rose Garden. "When I think about this boy, I think about my own kids."
On Sunday, after Zimmerman's acquittal, Obama issued a similar appeal. He said Martin's death was "a tragedy" for the family and for the country. But, he added, "we are a nation of laws, and a jury has spoken." He urged calm and summoned Americans to "widen the circle of compassion and understanding in our own communities."
At the White House on Monday, press secretary Jay Carney said Obama "has no opinion to express" about the Justice Department investigation or whether federal charges should be brought against Zimmerman.
"Cases are brought on the merits, and the merits are evaluated by the professionals at the Department of Justice," Carney said.
Some civil rights leaders said Obama was responding appropriately to keep from unduly influencing a Justice Department investigation.
"My heart would love to see him charge the barricade," the Rev. Al Sharpton, an activist and television host, said in an interview. "My mind tells me that would give us a moment of joy and a longevity of pain because it would undermine the potential merits of a federal case."
William Branigin contributed to this report.