ST LOUIS (CNN) -- The Ferguson, Missouri, police officer who killed Michael Brown says the teenager rushed at him full speed in the moments before the shooting, according to an account phoned in to a radio station and confirmed by a source with detailed knowledge of the investigation.
Meanwhile, a spokesman for the St. Louis County prosecuting attorney said Monday that evidence in the shooting death could be presented to a grand jury as early as Wednesday.
According to the account on St. Louis radio station KFTK, phoned in by a woman who identified herself as "Josie," the altercation began after Officer Darren Wilson rolled down his window to tell Brown and a friend to stop walking in the street.
When Wilson tried to get out of his cruiser, Brown first tried to push the officer back into the car, then punched him in the face and grabbed for his gun before breaking free after the gun went off once, the caller said.
Wilson pursued Brown and his friend, ordering them to freeze, according to the account. When they turned around, Brown began taunting Wilson, saying he would not arrest them, then ran at the officer at full speed, the caller said.
Wilson then began shooting. The final shot was to Brown's forehead, and the teenager fell two or three feet in front of Wilson, said the caller, who identified herself as the officer's friend.
A source with detailed knowledge of the investigation later told CNN the caller's account is "accurate," in that it matches what Wilson has told investigators.
If true, the account represents the first telling of events from the perspective of Wilson, whose shooting of Brown has touched off nightly protests and violence in the suburban St. Louis city.
A grand jury will hear testimony from witnesses and decide on whether to return an indictment in the case, Ed McGee, spokesman for the St. Louis County prosecuting attorney, said Monday, stressing there is "no time line on this case."
In addition to that proceeding, the Justice Department has opened a civil rights investigation into Brown's death. Attorney General Eric Holder will travel to Ferguson this week, to meet with investigators there.
Earlier Monday, an autopsy conducted for the family of Brown found no evidence that he struggled with Wilson before his death, according to the pathologist in charge of the examination.
Dr. Michael Baden said no signs of a struggle were revealed in his autopsy of Brown's body, conducted after an official examination by the St. Louis County medical examiner's office.
And forensics consultant Shawn Parcells, who assisted Baden, said the findings are consistent with witness reports that Brown may have been shot as he walked away and that he was shot again with his hands up.
Brown family attorney Daryl Parks said he was particularly concerned about gunshots that medical examiners hired by the family indicate came from behind and above.
"Why would he be shot in the very top of his head, a 6-foot-4 man?" Parks asked. "Makes no sense."
The autopsy results are the latest development in the investigation into Brown's death, which has resulted in nightly, sometimes violent, protests in Ferguson that have prompted Missouri's governor to declare a curfew and send in the state National Guard.
"Given these deliberate, coordinated and intensifying violent attacks on lives and property in Ferguson, I am directing the highly capable men and women of the Missouri National Guard ... in restoring peace and order to this community," Gov. Jay Nixon said in a statement. He told reporters the National Guard would have a "limited mission" to protect the command center in Ferguson.
'Nation of laws'
Speaking to reporters Monday, President Barack Obama called for calm in Ferguson, saying that violence undermines, rather than advances justice.
Gunfire, tear gas and Molotov cocktails Sunday night marked some of the fiercest clashes yet between police and protesters furious about the death of the unarmed teenager.
"We have all seen images of protesters and law enforcement in the streets. It's clear that the vast majority of people are peacefully protesting. What's also clear is that a small minority is not," said Obama, stressing he must "be careful" about not prejudging events.
"Let's see some understanding" rather than confrontation, and "let's seek to heal," he added.
"There's no excuse for excessive force by police or any action that denies people the right to protest peacefully. Ours is a nation of laws -- for the citizens who live under them and for the citizens who enforce them," Obama said.
The protests have also gained international attention. U.N. Secretary-General Ban Ki-moon issued a statement Monday on the events in Ferguson, saying that he "hopes local and federal investigations will shed full light on the killing" of Brown.
Ban called on authorities to ensure that people are able to assemble peacefully and urged law enforcement to abide by U.S. and "international standards in dealing with demonstrations."
The family autopsy found that Brown was shot at least six times, including two shots to his head. Three of the bullets may have re-entered his body, causing additional damage, Baden said.
One wound to his arm was consistent with a witness statement that Brown was walking away and appeared to jerk, as if shot, Parcells said. The wounds to his arm could have also have occurred while he had his hands up, possibly in a defensive posture, Parcells said.
One of the bullets entered his head and came out through his eye; another -- likely the fatal wound, Baden said -- struck Brown on the top of his head and caused irreparable damage to his brain.
Family attorney Benjamin Crump said Brown probably would have been either kneeling or bending forward when he was struck with those bullets.
Brown had abrasions on his face consistent with falling onto the ground, Baden said.
He cautioned that he needs access to autopsy results, including tests on Brown's clothes and X-rays, before making some conclusions.
But Crump said what it already revealed offered more than "ample" evidence to support Wilson's arrest.
"What does this autopsy say? That the witness accounts were true, that he was shot multiple times," Crump told reporters.
Devolution of protests
Another family attorney, Anthony Gray, implored protesters to remain peaceful.
"I can see that there is a very disturbing divide that is developing in our community," he said Monday. "This is not what we initially came to the community and called for."
As he spoke, the Missouri National Guard was preparing to deploy to Ferguson under orders from the governor to restore peace.
Nixon issued the order early Monday after what began as peaceful protests spiraled into disarray after two civilians were shot and injured, Missouri State Highway Patrol Capt. Ron Johnson said. He said those civilians were not shot by police.
Some protesters hurled Molotov cocktails at police, and several businesses were vandalized or looted despite the Brown family's call for calm.
"Based on these conditions, I had no alternative but to elevate the level of our response," Johnson said.
Officers fired tear gas into hundreds of protesters, including children, who were marching toward a police command post despite an impending midnight curfew.
Two children were treated and released for exposure to tear gas overnight at St. Louis Children's Hospital, according to a spokeswoman there.
Protester Lisha Williams challenged the notion that protesters provoked officers.
"That is a lie. It was no fight, it was no shots fired," she told CNN late Sunday. "All we did was march to the command center to fall to our knees and say, 'Don't shoot.' And they started shooting."
The clashes kept escalating, with St. Charles County sheriff's officials saying shots were fired in their direction.
St. Louis County police said most of the crowds had dispersed after the curfew went into effect at midnight. The curfew ended at 5 a.m. (6 a.m. ET). According to Gov. Nixon, there will be no curfew in Ferguson on Monday night.
But the anxiety remains. Children can't even go to school.
"Information we received from officials on the scene late Sunday evening has contributed to concerns we have about children walking to school or waiting for buses on streets impacted by this activity," the Ferguson-Florissant School District said on its Facebook page.
Accounts of exactly what happened when Wilson stopped Brown while the teen was walking down a street vary widely.
Witnesses said they saw a scuffle between the officer and Brown at the police car before the young man was shot.
Several witnesses said Brown raised his hands and was not attacking the officer.
Piaget Crenshaw said she was sitting in her home when she witnessed the shooting. She captured video of the aftermath, including images of Brown's body lying in the middle of the street.
"From it all initially happening, I knew this was not right," she told CNN's "New Day" on Monday.
"I knew the police shouldn't even have been chasing this young boy and firing at the same time. The fact that he got shot in the face, it was something that clicked in me, like no, somebody else needs to see this. This isn't right. I've got to record."
Crenshaw said Brown was running away from police and then turned around. She said that was when Brown was shot.
But police gave a different narrative, saying Brown struggled with the officer and reached for his weapon.
The officer has stayed out of the public spotlight, and more than 22,000 people have endorsed the "I Support Officer Wilson" Facebook page.
CNN's Steve Kastenbaum reported from Ferguson; CNN's Holly Yan and Michael Pearson reported and wrote from Atlanta. CNN's Melanie Whitley, Jennifer Duck, Steve Almasy, Dave Alsup, Jim Acosta, Mayra Cuevas, Evan Perez and Joe Sutton contributed to this report.
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