Sixteen immigrants died in U.S. government detention between December 2015 and April 2017, according to a new report by four civil rights organizations. Half of those deaths were preventable and caused in part by inadequate medical care, the report alleges.

Physicians with expertise in medical issues that occur in correctional facilities examined all but one of the 16 deaths reported by the federal government’s Detainee Death Reviews during that 17-month period. They found evidence of unreasonable delays, poor medical care and botched emergency response, according to the report, which was released just after midnight Wednesday by Human Rights Watch, the American Civil Liberties Union, the National Immigrant Justice Center and Detention Watch Network.

The eight deaths cited as preventable include one that occurred after a nurse at a California detention center refused to see one 54-year-old man, who was vomiting and having a heart attack, because she did not want to get sick herself.

A 51-year-old man died of bronchopneumonia after a delay in transferring him to a hospital from a New Mexico processing center, where nurses had noted his dangerously low oxygen levels for three previous days.

A 27-year-old man, who had mental health issues and had been identified as a suicide risk, was placed in solitary confinement at a Georgia detention center and was not regularly monitored. Nor were his antipsychotic medications adjusted. He hanged himself from his bunk bed.

Since March 2010, Immigration and Customs Enforcement has reported 74 deaths in its detention centers, and released reviews in 52 of those cases. In fiscal year 2017, a dozen people died in those centers, more than at any time since 2009, the report said.

ICE public affairs officials said in a statement Wednesday that it “takes very seriously the health, safety and welfare of those in our care, including those who come into ICE custody with prior medical conditions or who have never before received appropriate medical care. Any death that happens in ICE custody is a cause for concern.”

Each year, between 300,000 and 500,000 people are in the custody of ICE. When one dies, the agency’s health corps reviews the case medically and its Office of Professional Responsibility investigates it. The results of those examinations go to ICE senior management and the Department of Homeland Security’s civil rights office.

There is evidence that government officials have been aware of the problem. In late 2017, the Department of Homeland Security’s Office of Inspector General reported that after a survey, five centers were found to have undermined detainees’ rights including delayed and improperly documented medical care. It recommended that ICE field offices review the problems.

Even in cases in which failures of medical care did not contribute to deaths, the civil rights groups’ report said: “Of the 33 reviews for which we have sought independent medical reviews [over several years] . . . our experts said only three described what appeared to be adequate care.”

The organizations’ report said that the detainee reviews are not a representative sample but that the fact that the same problems have persisted over several years’ worth of reviews means that they “warrant immediate attention and action” by Congress, ICE and state and local governments that have authority over the agencies.

The four organizations called for reducing the number of people held in detention, removing the ICE medical providers from ICE supervision, appointing an independent medical oversight board and eliminating exemptions to federal disclosure laws that apply to private companies operating detention centers.