Tribune Broadcasting, the station’s parent company, said that Mr. Steincross would not face disciplinary action. Mr. Steincross started at Fox2 as a general assignment reporter in 1996 and became a morning newscaster in 1998, according to the station’s website.

“The Fox2 management team spoke to Kevin following the mistake, and we believe that it was truly inadvertent and does not reflect Kevin’s core beliefs,” the company said in a statement. “Kevin is extremely upset by the mistake and regrets it deeply. We do not believe additional disciplinary measures are necessary.”

But some people said the apology was not enough. The St. Louis chapter of the N.A.A.C.P. said on Twitter that it was “unacceptable and very disappointing” and called for Mr. Steincross to resign. Others on Twitter questioned how he could mix up the insult with Dr. King’s name.

During his lifetime, Dr. King’s last name was often swapped out for the epithet, both by avowed racists and others who claimed to have used it mistakenly. More recently, American broadcast journalists have made similar utterances in 2005, 2010, and 2014, the Rochester Democrat and Chronicle reported.

Heather Taylor, a sergeant in the St. Louis police force and president of the Ethical Society of Police, an association for African-American police officers in St. Louis, said the slur was so insulting because of the long history of its use against a man of towering stature.