JENA, La. (AP) — The Rev. Al Sharpton called Sunday for an investigation into a district attorney who has spearheaded the prosecution of a group of black teenagers on serious criminal charges stemming from a high school fight.

In a telephone interview from New York, Sharpton also said he and fellow civil rights leader Martin Luther King III would join ‘‘thousands of others’’ in converging on this northern Louisiana town on Sept. 20 — the day that Mychal Bell is to be sentenced on an aggravated second-degree battery conviction.

Bell faces up to 15 years in prison.

‘‘After that, if we need to, we’ll go to Baton Rouge and see the governor and the Legislature,’’ Sharpton said.

A year ago, three white students at Jena High School were suspended for hanging nooses in a tree on campus. One day earlier, a black student had sat under the tree, a traditional gathering place for whites.

In December, six black students — named the ‘‘Jena Six’’ by supporters — were arrested for beating up a white student. Five initially were charged as adults with attempted second-degree murder. Charges against three of them have since been reduced to aggravated second-degree battery. The sixth student faces charges in juvenile court.

The victim of the attack was treated at a hospital and attended a school function the same night.

Sharpton said protesters wanted to know why criminal charges were not filed against the students who hanged the nooses or against a young white man who pulled a shotgun on three black students at a convenience store. The three black teens who took the shotgun from him were arrested and accused of aggravated battery and theft.

Sharpton said he would ask for an investigation by the state attorney general and judicial oversight agencies into the actions of LaSalle Parish District Attorney Reed Walters, who has led the prosecutions.

A telephone message left at Walters’ residence was not returned Sunday. In the past, he has said he cannot comment because of the pending cases.

In Jena on Sunday, the Rev. Jesse Jackson spoke to about 200 people at a school where he urged the town’s 3,000 residents to come together to demand equal justice. Only about 350 residents of the town are black. In LaSalle Parish, only about 11 percent of the residents are black.

‘‘Why be fighting when we can turn to each other and find common ground?’’ Jackson said. ‘‘Jena is too small not to move together.’’

Jackson said he wanted to see white ministers and black ministers talking and white parents and black parents talking.

In reference to Bell, Jackson said that authorities called his tennis shoe ‘‘a weapon of mass destruction’’ and set his bond at $90,000. Bell was a star athlete who scored 16 touchdowns for Jena High’s football team last year, and was being courted by schools from UCLA to Louisiana State University.

Jackson said he planned to try to meet Monday with Bell, who remains in jail unable to post bond, as well as officials at the courthouse.

‘‘We’re calling on people. We want to meet with them. We want whites and blacks together. We don’t want this community to be permanently scarred,’’ Jackson said.

Jackson said the case was being watched closely internationally and its outcome would reflect on Jena, Louisiana and the United States. ‘‘American justice is on trial,’’ he said.

Jackson said he planned to participate in the Sept. 20 demonstration.