FERGUSON, Missouri (CNN) -

Attorney General Eric Holder will travel to Ferguson, Mo., this week, President Barack Obama said Monday. The Justice Department has opened a civil rights investigation into the shooting death of Michael Brown.

The president called for calm in Ferguson, in response to a week of sometimes violent demonstrations there in the wake of the shooting of the black teenager by a white police officer.

"Let's see some understanding" rather than confrontation, and "let's seek to heal," Obama said.

But earlier on Monday, the 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.

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.

"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.