Ferguson, Missouri (CNN) -- 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 tells CNN.
Federal investigators are joining the U.S. Department of Justice in assisting local authorities to garner the facts surrounding the killing 18-year-old Michael Brown, who was shot Saturday night.
Tensions have been high in the town of 21,000. Dozens took 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 told 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 on CNN on 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 had been 32 people arrested and 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 as they watched as young men kneel before them, their hands up to symbolize surrender.
But one officer can be heard on video yelling back, calling protesters "animals."
"Last night, everything lost control," Knowles said Monday on CNN's "New Day."
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.