For months, Bettersten Wade called the police in Jackson, Miss., desperate for any update or sign that detectives were making progress in tracking down Dexter, her 37-year-old son who left their home one day in March and vanished.

Each conversation ended in frustration. They never seemed any closer to finding him, she said.

And yet, records show that investigators for the Jackson Police Department knew exactly where Dexter Wade was. An off-duty police officer driving an SUV had struck and killed him on the same day that his mother last saw him, according to officials and coroner’s records. A deputy coroner said he was identified by a bottle of prescription medication he was carrying and through fingerprints.

Ms. Wade kept searching for more than five months, unaware that her son had been buried behind the local jail, in a ragged cemetery for the poor and unclaimed where graves are marked by small, numbered signs poking out of the dirt.

“All that time, they knew where he’s at,” Ms. Wade said of the Jackson police in a recent interview. “They knew who he was.”

Officials in Jackson, a city of 150,000, have described Mr. Wade’s burial and the long delay in informing his mother of his death as regrettable but honest mistakes, a matter of miscommunication within an overburdened police department.

But Ms. Wade was not inclined to give the police the benefit of the doubt. Four years ago, three officers had been charged with murder after they pulled her brother from his car and slammed him to the ground.

Ms. Wade wanted her son removed from the unmarked pauper’s grave. She wanted to be there when it happened, praying over his remains as they were raised from the dirt. And she wanted answers — from an independent autopsy and, she hoped, an investigation into her son’s death and how it was handled by the police.

“You can’t write it off as a mistake, you can’t write it off as a miscommunication,” Ms. Wade said. “I don’t exactly know why they did it. That’s what I want to find out.”

What happened to Mr. Wade drew national attention when NBC News reported on the case on Oct. 25. The next day, Mr. Wade’s family retained Ben Crump, the civil rights lawyer who handles high-profile cases of police misconduct across the country.

Activists and elected officials have now joined Ms. Wade in urging federal investigators to step in.

“The system owes Mr. Wade’s family an explanation for the callous manner in which his untimely death was mishandled,” Representative Bennie Thompson, a Democrat from Mississippi, said in a statement.

Chokwe Antar Lumumba, Jackson’s mayor, addressed Mr. Wade’s death in a speech last month, saying that he had not seen any evidence of police misconduct or “malicious intent.” Mr. Wade’s death, and the failure to tell his mother about it for months, “honestly was an unfortunate and tragic accident,” he said.

Still, lawyers for Mr. Wade’s family said that city and police officials have failed to provide any clarity about how his death was handled. The preliminary findings from an independent autopsy directly contradicted some of the statements from officials, including that he was not carrying identification.

“The fact that Dexter had a state identification card and several other identifying items,” Mr. Crump said in a statement, “shows us that there was a concerted effort to keep the truth and manner of his death from his family.”

The Hinds County coroner did not respond to messages seeking comment on Friday. A spokesman for the Police Department referred a reporter to the city’s spokeswoman, who said on Friday that the city was in no position to comment on what was in Mr. Wade’s possession.

“From the moment the coroner arrived on the scene of the accident until the moment Dexter Wade was buried, his body was in the custody of Hinds County and not the City of Jackson,” Melissa Faith Payne, the spokeswoman, said in a statement.

On March 5, Mr. Wade left home with a friend, his mother said. After about a week without hearing a word from him, she called the police to report him missing. It was unusual for him to leave home for a long period without sharing his plans with her. “He’d always let me know,” she said.

He had worked as a roofer, and he mostly stuck close to home, mowing the lawn and spending time with his children on visits. His mother knew he had been struggling with mental illness. He moved in with her after he was released from prison in 2017.

“After them six years, something just happened,” she said, referring to the time he spent incarcerated for an armed robbery conviction.

Coroner’s records show that at about 8 p.m. on March 5, Mr. Wade was hit while apparently trying to cross the southbound lanes of Interstate 55, southwest of downtown Jackson. Officials said the driver was an off-duty police officer in a department vehicle.

His death was ruled accidental by the Hinds County coroner. A toxicology screening found phencyclidine, known as PCP, and methamphetamine in his system.

The coroner’s office called Hinds Behavioral Health Services, which had prescribed the medication he was carrying. The clinic confirmed that Mr. Wade was a patient and provided the coroner’s office with his mother’s name and a phone number, according to notes attached to his death certificate.

A deputy coroner tried the phone number, records showed, but there was no answer. Ms. Wade said she did not receive a call. Then, the information was given to the police to try to reach Ms. Wade. The deputy coroner checked in with the police several times over the next few months to ask if anyone planned to claim the body, and there were no updates, according to notes in the coroner’s records.

On July 14, Mr. Wade was buried at the Hinds County Penal Farm.

Ms. Wade kept calling the missing persons investigators at the Police Department. Over the summer, the detective handling the case retired and a new investigator took over. Within two weeks, an officer showed up at Ms. Wade’s home, telling her that her son had died.

Ms. Wade was shocked and infuriated by the failure to inform her sooner. She believes she should have been familiar to the department: Not only had she hounded the agency to help find her missing son, she had also been a visible critic of the Jackson police after her brother, George Robinson, was fatally beaten by officers in 2019.

An internal investigation into Mr. Robinson’s death cleared the officers of wrongdoing before the county prosecutor revived the case and brought criminal charges. Charges against two of the officers were later dismissed; a third officer was convicted of manslaughter by culpable negligence and sentenced to five years in prison.

Ms. Wade was hopeful that exhuming her son could bring her a measure of peace.

A letter from the county’s lawyer confirmed that Mr. Wade’s body would be exhumed on Nov. 13 at 11:30 a.m. “It is the procedure of the coroner to exhume the body from the burial site in the presence of representatives of the Hinds County Sheriff Department, coroner’s office and the funeral home that will receive the body,” the letter said.

Instead, when Ms. Wade and employees of the funeral home showed up on Monday morning, his remains had been dug up hours earlier by an unsupervised public works crew.

Ms. Wade broke into tears as she stood in the pauper’s cemetery, dressed in black. She was crushed. She was angry. She was exhausted.

“I don’t matter,” she said. “It don’t matter what I think and what I feel. Nothing matters.”

The family’s lawyers said a “chain of custody” had been broken, denying them a chance to know how the remains had been handled. “We don’t know whether he was in a box, whether he was in a bag, what exact condition he was in,” said Dennis C. Sweet, one of the lawyers.

Mr. Wade’s body had not been embalmed, according to the preliminary findings of the independent autopsy released on Thursday. It showed blunt force injuries to his skull, ribs and pelvis, and his left leg was severed.

His wallet was in his front pocket. It had his credit card, his health insurance card and his state identification card, which listed the address where he had lived with his mother.

Emily Kask contributed reporting.