Advocates held a vigil for Rikers inmate Bradley Ballard on Thursday. View Full Caption DNAinfo/Eddie Small

CONCOURSE VILLAGE — Advocates for Bradley Ballard, a mentally ill Rikers Island inmate who died after being left in his cell a week, are trying to convince Bronx District Attorney Robert Johnson to prosecute jail staffers for his death.

Ballard, 39, died in his cell, where he had been left alone for seven days, in September 2013. The death has been ruled a homicide by the city's medical examiner's office.

The New York City Jails Action Coalition held a vigil for Ballard on Thursday outside of Bronx District Attorney Robert Johnson's office at 198 East 161st St. to persuade Johnson to press charges against the staff responsible for his death.

"If he would've died at the hands of another inmate, that inmate would have been prosecuted long ago," said Johnny Perez, an advocate at the Urban Justice Center, which is part of the coalition.

Roughly 15 people sang and chanted in support of Ballard at the vigil and held signs carrying messages like "No person deserves to die like this: isolated, ignored, alone," and "Rikers=death" written on a tombstone.

NYC JAC held its first vigil for Ballard on June 26, and the group has pledged to continue doing so every month "until justice is served."

The Department of Correction declined to comment on the ongoing investigation.

Terry Raskyn, a spokeswoman for the Bronx DA, said the process of Rikers cases getting referred to their office was "complex."

"This case came to our office relatively recently, and as with any Rikers case like this one, we are conducting our own thorough investigation," she said. "The investigation is ongoing, as we speak, and we cannot comment about the details or the results until it is complete."