"We are determined to meet division and confusion with understanding and compassion -- and also with truth," Holder said Monday at a convention of the Delta Sigma Theta, the nation's largest black sorority. "We are resolved, as you are, to combat violence involving or directed at young people, to prevent future tragedies and to deal with the underlying attitudes, mistaken beliefs and stereotypes that serve as the basis for these too common incidents."
"And we will never stop working to ensure that -- in every case, in every circumstance, and in every community -- justice must be done," he said.
A jury in Sanford, Fla., found Zimmerman, 29, not guilty of second-degree murder and manslaughter in the fatal shooting of 17-year-old Martin in February 2012. Zimmerman, a Hispanic-American Neighborhood Watch volunteer, claimed he shot Martin, who is black, in self-defense.
Holder told his audience that as the sorority was celebrating its founding, "we are also mindful of the pain felt by our nation surrounding the tragic, unnecessary shooting death of Trayvon Martin in Sanford, Fla."
The shooting sparked nationwide claims of racial profiling and demonstrations, as did Saturday's verdict.
"This tragedy provides yet another opportunity for our nation to speak honestly about the complicated and emotionally charged issues that this case has raised," Holder said. "We must not, as we have too often in the past, let this opportunity pass."
The Justice Department said it is reconsidering the filing of hate-crime charges against Zimmerman.
Holder said the department would still act "act in a manner that is consistent with the facts and the law."
"We are committed to standing with the people of Sanford, with the individuals and families affected by this incident, and with our state and local partners in order to alleviate tensions, address community concerns and promote healing," Holder said.
Meanwhile, Zimmerman will get his handgun back, his lawyer, Mark O'Mara told ABC News Sunday.
O'Mara said Zimmerman is entitled to get his Kel Tec 9 pistol and intends to rearm himself because there's "even more reason now."
"There are a lot of people out there who actually hate him, though they shouldn't," O'Mara said.
O'Mara also acknowledged the likely possibility of Martin's family filing a civil suit against Zimmerman, but said Zimmerman might file one of his own.
Groups, including the NAACP and the American Civil Liberties Union, have urged Holder to press a federal civil rights case against Zimmerman after a six-woman jury found Zimmerman accepted his claim he acted in self-defense.
The department has had an open investigation into Martin's death since soon after the shooting but put it on hold while the criminal trial was under way.
The NAACP began a petition Saturday asking the Justice Department to file civil rights charges against Zimmerman. The petition received such a massive response that it crashed the NAACP website, The Washington Post reported.
The Saturday night verdict prompted street protests in cities across the country, including Sanford, New York, Chicago, Washington, Atlanta, San Francisco, Oakland, Calif., and Los Angeles.
Nearly all were peaceful, but one of several protests in Los Angeles Sunday evening turned violent, with demonstrators throwing rocks and D-cell batteries at police, the Los Angeles Times reported.
President Obama, in a statement Sunday, called Martin's death a "tragedy" and urged the country to "stem the tide of gun violence."
He also appealed for calm amid "strong passions" the case elicited.