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FERGUSON, Mo. (CBS) -- The parents of slain suburban St. Louis teen Michael Brown appeared at a news conference in tears and demanding justice in the wake of the shooting Saturday that sparked vigils and demonstrations where thousands marched and prompted a federal investigation.

"We need justice for our son," said Brown's father, Louis Head, standing beside his wife Lesley McSpadden, who was barely able to stand behind a podium, slumped over with grief.

The parents were joined by their representing attorney Benjamin Crump, who represented the family of Trayvon Martin during the trial of George Zimmerman last year.

"I don't want to sugarcoat it, their baby was executed in broad daylight," Crump said of the 18-year-old who was unarmed when he was shot by a Ferguson, Mo., police officer. "That's why people are frustrated.

He said Brown's parents have called for calm as the investigation into the shooting proceeds.

"Respect how they raised him in the fact that he was a nonviolent kid," he said. He also called for any witnesses who recorded video or took pictures and do not feel comfortable giving them to local law enforcement that it could be given to the local NAACP, the family or the attorneys representing them.

"We want you in the public to know if you were a witness to what happened, don't feel intimidated come forward."

Meanwhile, the FBI has announced that it is investigating possible civil rights violations in the case.

Cheryl Mimura, a spokeswoman for the FBI's St. Louis field office, says the agency is monitoring the case and working with St. Louis County police. The FBI has stressed that it is not taking over the investigation from local officials.

In addition, Attorney General Eric Holder announced Monday that the Justice Department and the U.S. Attorney's Office will also launch a probe into Brown's death.

"The federal investigation will supplement, rather than supplant, the inquiry by local authorities," Holder said in a statement. "At every step, we will work with the local investigators, who should be prepared to complete a thorough, fair investigation in their own right. I will continue to receive regular updates on this matter in the coming days."

A candlelight vigil for Brown was held Sunday, and tensions erupted later that night. Nearly three dozen people were arrested after a crowd looted and burned stores, vandalized vehicles and taunted officers.

More background on the case:

FBI investigates shooting

Angry crowd appears after shooting

Some residents don't believe police

Black man dies in police chokehold

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