By Elisha Fieldstadt, NBC News

The FBI opened an independent probe into the fatal shooting of a 13-year-old boy in Northern California by sheriff's deputies who mistook his pellet gun for a real firearm, Sonoma County authorities announced on Friday.

Andy Lopez was shot seven times on Tuesday by two Sonoma County deputies after he ignored orders to drop a pellet gun that officers said resembled an AK-47 rifle, according to a Sonoma County Sheriff's Office statement. The shots rendered Lopez unresponsive and deputies handcuffed him after securing the pellet gun. He was pronounced dead at the scene after paramedics arrived, the statement said.

On Friday, Sonoma County Sheriff Steve Freitas announced that he welcomed the FBI's investigation in addition to the separate joint investigation by the Santa Rosa Police Department, Petaluma Police Department and the Sonoma County District Attorney's Office. Freitas said his office will cooperate fully with all parties.

"As a father of two boys about this age, I can't begin to imagine the grief this family is going through," Freitas said.

Freitas said the investigations would be "thorough and transparent," and on Thursday the sheriff's office released a timeline of the events leading up to the shooting. In the span of 10 seconds, the deputies made a call reporting a suspicious person, called for emergency assistance and notified dispatch of shots fired, according to the timeline.

FBI spokesman Peter Lee said it was the FBI's decision to open the investigation, saying that is common procedure in situations when the use of police force results in death. "We're going to examine whether or not a federal crime was committed," Lee told NBC News on Saturday.

The two deputies have been placed on administrative leave, according to the Sonoma County Sheriff's office, but members of the community are still outraged.

"He went down on the first shot. Why did they find it necessary to continue shooting," Viki Carolan asked NBC Bay Area. The Sonoma County coroner found two fatal bullet wounds in Lopez's body — one to his hip and one to his chest — and five additional entry wounds.

"It's the sheriff's fault," protesting residents chanted to the beat of drums on during a Thursday night demonstration, according to NBC Bay Area. On Friday, hundreds of teens gathered at Santa Rosa City Hall and marched to the Sheriff's office in a "Justice for Andy" protest.

Freitas thanked the people of Sonoma County for keeping protests and memorials "peaceful and respectful."

"My hope is that we can work with the community to help prevent a similar tragedy from happening in the future," he said.

The Associated Press contributed to this report.