Geneva Reed-Veal (l.) unveils a street sign bearing her daughter’s name during a ceremony renaming University Boulevard to Sandra Bland Parkway in Prairie View, Texas in April. A Prairie View officer says prosecutors silenced him after he attempted to come forward with his account of what occurred during Ms. Bland’s arrest during a traffic stop last summer.

A Texas police officer present during part of Sandra Bland’s arrest last summer has told the Houston Chronicle that the local district attorney’s office declined to allow him to testify before a grand jury about the controversial traffic stop.

Three days after a state trooper stopped Ms. Bland, who was black, for failing to use her turn signal when changing lanes as she drove to begin a new job at Prairie View A&M University, she was found hanging in her jail cell.

The case sparked an outcry, one of several incidents around the country that have stirred activists’ concerns about how police treat African-Americans.

The allegations by Prairie View officer Michael Kelley, who said he was told by local prosecutors that his career would suffer if he went public with what he saw of the traffic stop, come as a fractious debate has also enveloped the investigation into the death of Freddie Gray in Baltimore.

Ms. Mosby’s comments drew a swift rebuke from the union that represents the six Baltimore officers charged in the case, while the prosecutor in Waller County, Texas, similarly called Officer Kelley’s account “fictional.”

Waller County District Attorney Elton Mathis Jr. noted that Kelley was later indicted for unlawfully arresting a local city councilman.

“I unequivocally state that he never approached me, my first assistant, or any member of my staff with any such information,” Mr. Mathis said in a statement emailed to the Chronicle. “His job was never threatened by me or my staff, and I barely knew who he was before he was indicted.”

But Kelley said the unrelated unlawful arrest charge was leveled against him but none of the other officers involved.

“I was not a target until I started running my mouth and sticking up for Sandra Bland,” Kelley told the Chronicle.

Concerns about a “blue wall of silence” that discourages officers to come forward need to be remedied in the hiring process, Kami Chavis-Simmons, a former US attorney who directs the criminal justice program at the Wake Forest University School of Law, told The Christian Science Monitor this week.

“Are they bringing in people who have integrity, and are independent, and who are going to be willing to blow the whistle when they see wrongdoing?” she asked.

Kelley told the Chronicle that when he arrived on the scene near Prairie View A&M on July 10, 2015, Bland was already in the back seat of a squad car covered in grass and dirt. Her forehead showed signs of bruising, he said.

Trooper Brian Encinia, who was shown on a dashboard camera video stopping Bland and getting into a confrontation with her in which he threatened to “light you up” with a Taser, told Kelley at the time he didn’t know what charge he would use to book Bland into the jail.

Kelley also told the Chronicle he heard Bland tell Encinia that she suffered from epilepsy, to which the trooper responded “Good.”

But after Bland’s death, he says a Texas Ranger initially told him and another officer to make a report of what he had seen, then told them not to sign or copy the reports, an instruction he told the Chronicle raised “red flags on my end.”

A special prosecutor who investigated Bland’s detention and death, however, criticized Kelley’s assertions, calling them “outlandish.”

“Why would a police officer who’s in the know, who knows the system inside and out, contact the DA when there were special prosecutors involved and they were handling the case?” Darrell Jordan told the Chronicle.

“Anybody on the street could have said they wanted to testify,” Mr. Jordan said. “We wanted anybody…. He never, never, never, never (contacted us).”

Bland’s death was ruled a suicide, but Kelley’s account and other allegations have emerged in a federal wrongful death suit filed by her mother.

A lawyer representing Bland’s mother, Geneva Reed-Veal, said last week that a former jail guard told him under oath that he had falsified jail logs, the Chronicle reports.

A grand jury declined to indict anyone from the jail in December, but later indicted Trooper Encinia on a single perjury charge, which is pending. He was fired in March.