FERGUSON, Mo. — President Barack Obama on Monday dispatched the attorney general to personally oversee the government’s response to the fatal police shooting 10 days ago of an unarmed black teenager, the latest step in an extensive federal investigation that was expanding even as National Guard troops moved onto the streets of this St. Louis suburb.

Attorney General Eric Holder will meet Wednesday in Ferguson with some of the FBI agents and prosecutors who have already interviewed more than 200 people as they scour the area where 18-year-old Michael Brown was killed by a white police officer. Holder pledged “the full resources” of his department to investigate Brown’s death, which has triggered unrest so severe that Missouri’s governor on Monday called in the state’s National Guard.

Behind the scenes, the administration worked to reassure some in the civil rights community that the nation’s first black president sees the Ferguson crisis as an important moment. In a conference call Monday with civil rights groups, Holder and White House adviser Valerie Jarrett – perhaps Obama’s closest confidant – said the case is a top priority.

“We are working tirelessly,” Holder said, according to people on the call, describing how federal investigators have located more than 200 people in a week. He said the investigators are trying to determine if there is enough evidence that Ferguson police officer Darren Wilson used excessive force and deprived Brown of his civil rights to support a federal criminal prosecution.

A senior federal law enforcement official said in an interview that the FBI’s St. Louis field office has spoken to residents with information about the shooting who had not talked to local investigators. The federal probe, the official said, is “moving swiftly.”

The developments came on a day when Obama addressed the chaos in Ferguson in starkly emotional terms at the White House, and Missouri Gov. Jay Nixon lifted the overnight curfew he had imposed on the city, saying the imminent arrival of the National Guard would help restore order.

The day also featured revelations from competing autopsies of Brown’s body – the official St. Louis County autopsy and another one requested by his family – both of which concluded that the teenager had been shot six times. Holder said a third autopsy was conducted Monday by the U.S. military.

Nixon had ordered the midnight-to-5 a.m. curfew two days earlier after a week of clashes between demonstrators and police following Brown’s death on Aug. 9. He brought in National Guard troops after the most chaotic night yet Sunday, one marked by protesters shooting and throwing molotov cocktails at police, and officers deploying tear gas.

The arrival of the Guard troops evoked images of the civil rights movement and the Vietnam War protests of the 1960s – when they were deployed to help integrate institutions in the South and restore order in burning cities – though the Guard is better known recently for its full-time roles in Iraq and Afghanistan.

Monday, the Guard’s role contributed to what appeared to be the latest example of federal-state tensions in the response to Brown’s death, which is also being investigated by state officials. A White House official said Nixon did not notify the administration before announcing his decision to call up the Guard.

At his White House news conference, Obama was notably cool in his response and offered only lukewarm support. The president said he had told Nixon in a phone call Monday that the Guard should be “used in a limited and appropriate way,” and he told reporters that he will be watching in the coming days “to assess whether in fact it is helping rather than hindering progress in Ferguson.”

Obama was more emotional in talking about Brown’s death and what it shows about the state of race relations in America five years after he took office, even using the occasion to proudly reminisce about how he passed legislation dealing with racial profiling back when he was serving in the Illinois legislature.

While acknowledging that “there are young black men that commit crime,” the president lamented statistics showing that “you have young men of color in many communities who are more likely to end up in jail or in the criminal justice system than they are in a good job or in college.”

“As Americans, we’ve got to use this moment to seek out our shared humanity that’s been laid bare by this moment,” he said. “The potential of a young man and the sorrows of parents, the frustrations of a community, the ideals that we hold as one united American family.”

As he has before, the president said that while most demonstrators are acting peacefully, “a small minority of individuals are not. While I understand the passions and the anger that arise over the death of Michael Brown, giving in to that anger by looting or carrying guns and even attacking the police only serves to raise tensions.”

Civil rights leaders said they were pleased by the administration’s escalating response, saying it was a step forward for Obama’s commitment to achieve racial justice. “Having the attorney general visit the site of an ongoing investigation is extra rare,” said NAACP President Cornell William Brooks, who has been in regular touch with the White House over the past week. He added that the “U.S. government’s pursuit of justice for this family is huge.”

Holder on Sunday ordered an independent federal autopsy of Brown’s body, and he said Monday that he is “confident this additional autopsy will be thorough and aid in our investigation.” While conducting three autopsies of the same body is highly unusual, experts said experienced forensic pathologists would have no trouble performing an accurate additional examination. They said, however, that it’s unlikely that the subsequent examinations would uncover substantial factual differences.

Indeed, the autopsy by St. Louis County Chief Medical Examiner Mary Case and the private one sought by Brown’s family yielded results that “sound consistent,” according to Benjamin Crump, an attorney for Brown’s family.

Case’s review, released to state prosecutors late Friday, found that Brown had six gunshot wounds to the head and chest and was shot from the front, people familiar with the autopsy said. It also showed that Brown had marijuana in his system, they said.

Two forensic pathologists hired by Brown’s family said Monday that their autopsy also showed that he was hit by at least six shots, one of which struck him at the top of the head and traveled downward through the brain. They said two wounds appeared to be “re-entry” wounds.

A wound to Brown’s right arm could have occurred when he was putting his hands up, as witnesses have claimed, or when he was putting his arm across his body in a defensive manner, the pathologists said at a news conference. They said that there were no signs on Brown’s body of a struggle and that he did not appear to have been shot at close range because no gunshot residue was found on his body.

An attorney for the Brown family said the autopsy showed that the teenager was “trying to surrender” when he was shot, but the forensic pathologists – Michael Baden and Shawn Parcells – said they could not determine whether that was the case.

Baden, a former longtime medical examiner in New York City, said Brown could have survived all of his gunshot wounds except the one to the top of his head that went through his brain.

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