Staffing shortages at the prison where financier Jeffrey Epstein was found dead left gaps in his supervision, said the union for the facility's employees.
The justice department and the FBI have both launched investigations into the cause and circumstances of his death in New York.
Epstein, 66, was facing sex-trafficking and conspiracy charges, which carried jail sentences of up to 45 years.
A post mortem examination was performed on Sunday.
New York City's chief medical examiner, Barbara Sampson, said more information was needed before the cause of death was determined.
A private pathologist observed the examination at the request of Epstein's representatives, Dr Sampson added.
Epstein's body was found in his cell early Saturday morning.
Guards at New York's Metropolitan Correctional Center had been forced to work overtime to make up for the staffing shortages, according to the union representing the prison guards.
A hiring freeze by the Trump administration has left thousands of staff vacancies across the Bureau of Prisons, creating "dangerous conditions" for both staff and inmates, said Eric Young, president of the American Federation of Government Employees Council of Prison Locals in a statement.
The remaining officers are regularly forced to work 70- and 80-hour work weeks, Mr Young said.
He also described a tactic to address the shortages called "augmentation", which allows teachers, nurses, clerical workers and other support staff to fill in for correctional officers.
"The low pay, understaffing, augmentation, and mandatory overtime have created an environment inside our prisons where something like this is even possible," he added.
One of the corrections officers was reportedly on his fifth straight day of overtime shifts, while another guard had been forced to work overtime, Serene Gregg, president of the AFGE Local 3148, told the Washington Post newspaper.
"If it wasn't Mr Epstein, it would have been somebody else, because of the conditions at that institution," she told the newspaper.
"It was only a matter of time for it to happen. It was inevitable. Our staff is severely overworked."
Ms Gregg said she has long complained about the work conditions at the facility.
The guards failed to follow several protocols leading up to Epstein's death, according to the New York Times.
Epstein, who had been placed on suicide watch after an apparent suicide attempt last month, was supposed to have a cellmate and checked in on by a guard every 30 minutes. Mr Epstein was reportedly left alone early on Saturday after his cellmate was transferred.
On Monday French government ministers also called for an investigation into Epstein, saying a US probe into the accused child sex trafficker had revealed links between Epstein and France.
What happens to the case against him?
Epstein - a convicted paedophile - was arrested on 6 July on new sex-trafficking charges. The indictment alleged that he paid underage girls to perform sex acts at his Manhattan and Florida mansions between 2002 and 2005.
According to the charges - which Epstein denied - the girls, some as young as 14, were given hundreds of dollars for sex acts.
Hundreds of pages of court documents unsealed on Friday - one day before Epstein's death - included new details of the sexual abuse claims, including allegations by a woman that she was forced to have sex with Epstein's powerful friends.
The documents shift the focus from Epstein to some of his high-profile associates, namely Ghislaine Maxwell, his former girlfriend.
Virginia Giuffre, an alleged victim of Epstein, claims Ms Maxwell recruited her as a masseuse for the hedge fund manager at age 15. In the same documents, Ms Giuffre alleges that Ms Maxwell introduced her to Britain's Prince Andrew, and encouraged her to have sex with him.
Buckingham Palace has said that "any suggestion of impropriety with underage minors is categorically untrue". The allegations were struck from the court's record in 2015.
Ms Maxwell has also denied wrongdoing.
Without Epstein to stand trial, legal experts told CBS News that federal prosecutors were likely to dismiss the case against him.
Lisa Bloom, an attorney for several women who claim they were abused by Epstein, told CBS that she planned to file civil litigation against Epstein's estate.
US Attorney General William Barr said on Monday: "Mr Epstein's death raises serious questions that must be answered". He also called for a "thorough" investigation.
What questions remain?
The death of the high-profile financier spurred a flurry of unsubstantiated conspiracy theories online.
But without the results from the post-mortem examination, questions still loom regarding the precise cause of death.
A city official told the New York Times that Dr Sampson was "confident" the cause of death is suicide by hanging, but she was awaiting further information from law enforcement.
Revelations that Epstein was left unsupervised after an apparent suicide attempt last month have also raised questions.
Who is Jeffrey Epstein?
Born and raised in New York, Epstein worked as a maths and physics teacher in the 1970s before moving into finance, creating his own firm: J Epstein and Co.
The company reportedly managed assets of clients worth more than $1bn (£800m). Epstein soon began spending his fortune - including on a mansion in Florida, a ranch in New Mexico, and reputedly the largest private home in New York.
But the specifics of Epstein's work - including his client list - remained largely shrouded in secrecy. Reports of Epstein's actual wealth varied, with his Virgin Islands-based firm generating no public records.
He was better known for his famous circle of friends and associates. Epstein was tied to US President Donald Trump, former President Bill Clinton, Prince Andrew, actor Kevin Spacey and high-profile lawyer Alan Dershowitz.
He first came under scrutiny from law enforcement in 2005, when the parents of a 14-year-old girl told police in Florida that Epstein had molested their daughter at his Palm Beach home. A police search of the property found photos of girls throughout the property.
But prosecutors forged a deal with the financier in 2008 and Epstein avoided federal charges - which could have seen him face life in prison. Instead, he received an 18-month prison sentence, during which he was able to go on "work release" to his office for 12 hours a day, six days a week. He was released on probation after 13 months.