The FBI is looking into whether the former South Carolina sheriff's deputy committed a crime. And the FBI and U.S. attorney's office are looking into whether the student's civil rights were violated.
Richland County Sheriff Leon Lott said he fired Fields because the deputy threw the student at Spring Valley High School.
"The fact that he picked the student up and he threw the student across the room -- that is not a proper technique, and should not be used in law enforcement," Lott said.
But the former deputy's attorney said his client acted lawfully.
"We believe that Mr. Fields' actions were justified and lawful throughout the circumstances of which he was confronted during this incident," attorney Scott Hayes said.
Sheriff: The incident started with the student
At least three videos have surfaced of the violent arrest at Spring Valley High School. The sheriff said one of the videos shows the girl attacking the officer before the arrest.
"When the officer puts his hands on her initially, she reaches up and she pops the officer with her fist," he said.
Still, after watching all the footage, Lott told reporters that he "wanted to throw up."
Richland County has 87 school resource officers, he said, and the "actions by Deputy Fields was not typical of the job I expect them to do."
The videos show the officer standing over the student, seated at her desk. He puts his arm near her neck, then yanks her backward. The chair tips over, and the student crashes backward onto the floor.
But Fields didn't let go, lifting her slightly off the ground. She flies out of her desk and slides several feet across the floor.
The sheriff said he spoke with Fields after his firing.
"He regrets it," Lott said. "He's been at that school for seven years. He loves those students. ... He wishes it never happened this way. This wasn't his intent."
Student: It started with a cell phone
The 16-year-old student was arrested on a charge called "disturbing schools." A classmate, Niya Kenny, 18, was also arrested on the same charge.
Kenny said the incident started when her math teacher told the other girl to give up her cell phone. The girl refused and defied orders from the teacher and an administrator to leave the classroom.
That's when Fields
was called in, Kenny said. The school resource officer asked the girl to leave the classroom with him.
"He grabbed her arm, and he put his arm around her neck at first. So that's why you actually see her -- if you get the right video -- then you'll see her trying to swing at him," Kenny said.
"And at that point, he just flipped the desk back and grabbed her out of it and threw her. And that's when you see her rolling across the floor."
Kenny said she and other classmates had their cell phones recording because of the officer's reputation.
"When he came in the classroom, I immediately told my classmates, 'Get your phones out, get your phones out. I think this is going to go downhill.' And it did."
Her attorney, Simone Martin, said she's been told "by a number of the students that he is referred to as Officer Slam as opposed to Officer Fields
. And that's telling."
Kenny was arrested and accused of disturbing school after yelling and cursing at the officer, according to an incident report.
Sheriff criticizes law on disrupting school
CNN law enforcement analyst Harry Houck said the officer shouldn't have been called in to deal with the student in the first place.
"Too often, these teachers in these schools are calling on the cops because they have a disruptive student in the classroom," he said. "This is not a cop's job."
But South Carolina has a law that muddles the role of school resource officers, the sheriff said.
"Unfortunately, our Legislature passed a law that's called 'disturbing schools,' " he said.
"If a student disturbs school -- and that's a wide range of activities, 'disturbing schools' -- they can be arrested. Our goal has always been to see what we can do without arresting the kids. We don't need to arrest these students. We need to keep them in schools."