The attorney general vowed to combat “mistaken beliefs and stereotypes” that lead to violence like the shooting of Trayvon Martin. (The Washington Post)

With the acquittal of George Zimmerman continuing to reverberate nationwide, Attorney General Eric H. Holder Jr. said Monday he shares concerns about “the tragic, unnecessary shooting death” of an unarmed black teenager in Florida last year, and he vowed to pursue a federal investigation into the matter.

In a speech at the social action luncheon of the Delta Sigma Theta sorority, Holder pledged that the Justice Department would “continue to act in a manner that is consistent with the facts and the law” and would work to “alleviate tensions, address community concerns and promote healing” in response to the case.

“We are determined to meet division and confusion with understanding and compassion — and also with truth,” he said. “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.”

Holder appeared at the Washington Convention Center, where the predominantly African American sorority is celebrating the 100th anniversary of its 1913 founding by 22 women at Howard University in the District.

On Tuesday, Holder travels to Florida to address the NAACP’s annual convention in Orlando, a short drive from the Sanford, Fla., courthouse where Zimmerman was found not guilty Saturday night of charges that he murdered unarmed teenager Trayvon Martin last year.

At the White House, press secretary Jay Carney said President Obama would have no comment about the Justice Department investigation. Asked whether Obama is feeling pressure to weigh in, Carney said, “Cases are brought on the merits, and the merits are evaluated by the professionals at the Department of Justice.”

Amid the sorority’s celebration, Holder said in his speech, “we are also mindful of the pain felt by our nation surrounding the tragic, unnecessary shooting death of Trayvon Martin in Sanford, Florida, last year, and we are cognizant of the fact that the state trial reached its conclusion over the weekend.”

Noting that “the Deltas are deeply, and rightly, concerned about this case,” he added, to applause: “The Justice Department shares your concern — I share your concern — and, as we first acknowledged last spring, we have opened an investigation into the matter.”

He said the tragedy provides an opportunity for the nation “to speak honestly about the complicated and emotionally-charged issues that this case has raised.” This opportunity “to better understand one another and to make better this nation we cherish” must not be allowed to pass, Holder said.

According to an Obama administration official, Holder will speak about the case in greater depth in his speech to the NAACP in Florida.

The verdict triggered objections from civil rights organizations and scattered demonstrations across the country by protesters who want Zimmerman held accountable for Martin’s death.

Zimmerman, 29, who has a white father and Hispanic mother, claimed self-defense in the shooting of Martin, a 17-year-old African American he saw walking through his gated community and followed on a rainy night in February 2012. Zimmerman said he fired after Martin punched him, knocked him down and started pounding his head into a concrete walkway. Prosecutors said Zimmerman profiled the teenager and initiated the confrontation that resulted in his death.

NAACP leader Benjamin Jealous launched a petition late Saturday to urge the Justice Department to file civil rights charges against Zimmerman. The petition received such a massive response that it crashed the NAACP Web site.

Hours later, the Justice Department released a statement saying that its civil rights division still has an open investigation into Martin’s death, a probe it launched in March 2012. Working with the FBI, federal prosecutors are reviewing evidence to see whether the case can be prosecuted, the statement said.

It pledged that the department would investigate the “facts and circumstances” of the shooting and take “appropriate action.” The statement said federal prosecutors would “determine whether the evidence reveals a prosecutable violation of any of the limited federal criminal civil rights statutes within our jurisdiction, and whether federal prosecution is appropriate in accordance with the department’s policy governing successive federal prosecution following a state trial.”

Among those calling for Justice Department action were three Democratic congressmen from New York: Charles B. Rangel, Hakeem Jeffries and Gregory W. Meeks. They were scheduled to appear outside the federal district courthouse in Manhattan on Monday afternoon to urge an investigation of possible civil rights violations.

In Los Angeles, police began making arrests early Monday after about 80 protesters blocked traffic in Hollywood, the Associated Press reported. Demonstrators also blocked traffic in Oakland on Sunday night, and a smaller group began vandalizing businesses and throwing rocks and bottles toward police, AP said.

Other rallies held Sunday in cities across the country were largely peaceful, news agencies reported.

Civil rights groups, including an organization headed by the Rev. Al Sharpton, plan to hold prayer vigils and rallies Saturday in at least 100 cities to protest the Zimmerman verdict and demand that federal civil rights charges be filed.