A racial justice activist with a wide following on Twitter denied conservative bloggers’ claims on Wednesday that he had lied about his race and about being the victim of a racially motivated attack.

The activist, Shaun King, 35, rose to prominence in the Black Lives Matter movement after the fatal police shooting of Michael Brown, an unarmed teenager, in Ferguson, Mo., in August 2014. He has said that his mother is white and his father is black and that the assault fueled his passion for helping people.

But conservative bloggers accused him of lying about both, pointing to a police report from an incident that Mr. King describes as his brutal beating by a white mob. In the report, dated March 1, 1995, Mr. King’s race is marked as white, his injuries are described as minor, and only one other student is identified as being involved.

Keith Broughton, the investigating detective, said in a telephone interview on Wednesday that he had not asked Mr. King about his race but filled out the form based on the observation of the student’s light skin and white mother. He said he interviewed six witnesses put forth by the school’s principal, including a teacher who broke up the fight. They all described it as a one-on-one altercation, he said.