An inmate in Denver’s Downtown Detention Center was killed Monday morning during a fight with another inmate, the Denver Police Department said.

No deputies were involved in the fight, sources told The Denver Post.

The names of inmates involved, cause of death or circumstances surrounding the altercation have not been released by the Denver Police Department, which is investigating the death.

“The incident that occurred at the Downtown Detention Center this morning remains under investigation, and there are limited details that can be shared at this time,” Sheriff Patrick Firman said in an emailed statement. “However, we can confirm that the facility was appropriately staffed and the housing unit where the incident occurred was below capacity. Employees and the inmates who witnessed the incident have been offered wellness assistance, and we will continue to support their well-being.”

The death comes at a time when the Denver Sheriff Department has been criticized by its deputies’ union for increasing violence inside the city’s two jails.

Last year, the sheriff’s department reported 513 inmate-on-inmate assaults, six times the number of assaults reported four years ago, according to a Denver Post report in April. Assaults on deputies also have been on the rise.

While Firman has acknowledged the increasing violence, he has said a reason for increasing violence is unknown. Some observers attribute the violence to crowded jails, more inmates with mental illnesses and to deputies who have become more vigilant in filling out reports.

Meanwhile, the deputies’ union has warned the City Council that deputies and inmates are in danger because of the violence. At a rally in February, one veteran deputy called the Downtown Detention Center “a dangerous powder keg that is ripe for another tragedy.”

Between 2011 and 2015, 20 people died in Denver’s jails, a Denver Post investigation found this year. That was the highest number of deaths among the state’s 56 county jail systems; Denver’s is the largest jail in the state.

The last in-custody death in a Denver jail occurred Jan. 17, when an inmate died from a medical complication, said Daelene Mix, a spokeswoman for the Denver Department of Safety.

The latest inmate fatality falls on the seventh anniversary of the death of inmate Marvin Booker, who died at the hands of Denver Sheriff Department deputies on July 10, 2010. His family held a rally Monday evening at the downtown jail to urge the district attorney to reopen a criminal investigation into his death, and prayers were offered for the inmate who died Monday.

Inmate-on-inmate violence also has been costly in Denver.

In 2011, inmate Jamal Hunter was attacked by other inmates, who beat him, attempted to suffocate him and poured scalding water on his genitals. The city paid Hunter a $3.25 million settlement after he and his attorneys said a jail deputy turned out lights and looked away as Hunter was given a beatdown after being called a snitch by the deputy.

After Monday morning’s death, community activists who follow law enforcement called for a thorough investigation and vowed to continue to be watchdogs over the city’s jails. They, too, have attended rallies and spoken to a City Council committee about conditions inside the jail.

“While not all of the facts are out, we fully expect a transparent investigation, not only of the direct actors involved but also an up-the chain inquiry,” the Colorado Latino Forum said in a news release. “Ultimately, the buck stops with leadership at the top, and they too must be held accountable.”