FERGUSON, Mo. -- The FBI has officially opened an investigation into the shooting of a teenager by a police officer in the St. Louis suburb of Ferguson, Missouri, a U.S. law enforcement official said.
Federal investigators are joining the U.S. Department of Justice in assisting local authorities to discover the facts surrounding the killing of 18-year-old Michael Brown, who was shot Saturday night.
Tensions have been high in the town of 21,000. Dozens took to the streets Monday to march and chant, "No justice, no peace!"
A vigil for the teen devolved into chaos Sunday when violence and looting broke out among protesters.
Witnesses to Brown's shooting said he had been unarmed and had his hands in the air.
Authorities tell a different story. They say the police officer tried to get out of his vehicle just before the shooting, but Brown pushed him back into his car, said St. Louis County Police Chief Jon Belmar.
Brown "physically assaulted" the officer, Belmar said, and the teen tried to get the officer's weapon.
Brown was shot about 35 feet from the vehicle, the chief said, declining to provide more details.
"The genesis of this was a physical confrontation," Belmar said, adding that his department has been called in to conduct an independent investigation.
Ferguson Police said its cars are not equipped with dashboard cameras.
Shell casings collected at the scene were from the officer's weapon, Belmar said.
Beyond Ferguson, social media messages were on fire Monday as people are posted strong opinions about the killing. #MichaelBrown and #Ferguson were trending topics.
Mayor calls for calm
"Obviously, the events of last night are not indicative of who we are," Ferguson Mayor James Knowles said Monday morning, adding that the chaos that erupted Sunday night was "not constructive" and was only "bringing down the community."
St. Louis County Police said that 32 people were arrested and that shots were fired at police.
Flanked by several from the community Sunday, Michael Brown's mother, Lesley McSpadden, was emotional as she shouted into a television reporter's microphone.
"You took my son away from me! You know how hard it was for me to get him to stay in school and graduate? You know how many black men graduate? Not many!" she said. "Because you bring them down to this type of level where they feel they don't got nothing to live for anyway! (They feel) they gonna try to take me out anyway!"
Others who had gathered shouted at police.
"We will stay out here as long as you are!" they screamed at officers.
Many of the officers appeared stoic, watching young men kneel before them and raise their hands up to symbolize surrender.
But one officer can be heard on video yelling back, calling protesters "animals."
Some in Ferguson have demanded that the name of the officer who shot the teenager be released.
On Monday afternoon, Ferguson Police Chief Thomas Jackson said he plans to release the name in the next 24 hours. Authorities want to make sure the officer is in a safe location, he said.
"Last night, everything lost control," Knowles said Monday.
He was asked about the officer who called protesters "animals."
"The officers did their best. They're only human," Knowles responded, adding that not every police officer present was from the Ferguson department.
Sunday's gathering became more intense as some people broke windows at a store and began taking things from it. They threw rocks and bottles. Gunshots rang out.
Antonio French, an alderman in St. Louis, said a QuikTrip gas station was looted and an ATM dragged out.
"This QuikTrip is where things started (Saturday) with this case, based on various accounts," French said.
The slain teenager and a friend were "accused of stealing gum from the store or some sort of cigarettes," the alderman said.
Knowles said he wants to let the independent investigation into Brown's death take its course. He plans to meet with Brown's parents soon and will meet this evening with clergy in Ferguson and African-American leadership in the town.
Whatever the investigation's findings, "we will deal with that," he said.
On Monday morning, a medical examiner was expected to issue a ruling on how many times Brown was shot.
The medical examiner for St. Louis County, Mary Case, said Monday afternoon that the autopsy was complete. She would not give details and said St. Louis County police will release information. Toxicology results on Brown's body are pending.
On the number of times Brown might have been shot, Belmar has said, "It was more than just a couple."
Witnesses said Brown did nothing to instigate the shooting and appeared to be surrendering when he was killed. Brown was spending the summer in the neighborhood with his grandmother Desuirea Harris, she told KMOV.
"My son just turned 18 and graduated high school and he didn't bother nobody," his mother, Lesley McSpadden, told KSDK.
Brown was supposed to start classes at Vatterott College on Monday, she said.
"People have a lot of anger and are frustrated," French said. "They don't have recourse in the system, and it happens often in this country, and it has boiled over. I think people are angry and looking for a reason to let it out tonight."
Family retains Trayvon Martin lawyer
"We don't know what happened, and there are lots of conflicting stories," Knowles said. "Unfortunately, there will have to be some time taken to understand what happened. Hopefully, we will get to an understanding, and justice will be served."
The officer who shot Brown is on paid administrative leave during the investigation and will be available to talk to county homicide detectives.
He has been with the force for six years and will be required to undergo two psychological evaluations before returning to duty, Belmar said.
U.S. Attorney General Eric Holder has instructed the Justice Department's civil rights division to monitor the developments in the case.
Benjamin Crump, the lawyer who handled Trayvon Martin's case, will represent the family. Martin, 17, was killed in 2012 by a Florida neighborhood watch organizer, who was acquitted of murder charges.
Crump said Monday that Brown's family is "devastated" and don't believe that their son got into a physical confrontation with police.
They want the Justice Department to get involved in the case, the attorney said.
"Their son was doing all the right things," Crump said. "Graduating from high school, never been in trouble. And for this to happen, for him to be killed in broad daylight ... they want answers just like everybody else in the community."