NEW YORK — Prosecutors announced Wednesday that they won’t bring criminal charges in the death of a prison inmate whose family alleges he was beaten and thrown down stairs by guards known as the Beat Up Squad.

The family of inmate Samuel Harrell had alleged in a lawsuit he died in April 2015 after being assaulted by guards at the Fishkill Correctional Facility in Beacon, 70 miles (113 kilometers) north of New York City.

Prosecutors said in a joint release that they were closing the probe because of insufficient evidence. Manhattan Acting U.S. Attorney Joon H. Kim and Dutchess County District Attorney William V. Grady said the decision was reached after a thorough investigation.

The prosecutors met Wednesday with Harrell’s family and described the results.

Kim said his prosecutors needed to establish beyond a reasonable doubt that a law enforcement officer willfully deprived Harrell of a constitutional right.

“Neither accident, mistake, fear, negligence nor bad judgment is sufficient to establish a federal criminal civil rights violation,” the release said.

Harrell’s family said it was “deeply troubled” by the decision not to bring charges against the guards. It said an autopsy unambiguously linked Harrell’s death to “the brutal actions of these officers.”

“In our opinion, there was clear criminal wrongdoing by the Corrections Officers involved in Mr. Harrell’s death,” one of the family’s attorneys, Luna Droubi, said in an emailed statement. “The question of criminal liability should have been brought to and decided by a jury.”

In the state investigation, facts surrounding the physical altercation between guards and Harrell were not enough to prove that his death resulted from an intentional act, a reckless act or a criminally negligent act, prosecutors’ release said.

The prosecutors noted that there was no video evidence of the altercation between the 30-year-old Harrell and the guards and that eyewitness accounts by inmates and others were inconsistent and contradictory.

The prosecutors also cited inconclusive medical evidence of excessive force.

The medical examiner’s report concluded that the manner of Harrell’s death was “homicide,” but the cause of death was listed as “cardiac arrhythmia due to hypertensive cardiovascular disease following physical altercation with corrections officers.” The report said Harrell suffered from an enlarged heart and no broken bones or other serious injuries were found.

In a 2015 lawsuit, Harrell’s family said prison guards tried to cover up Harrell’s beating death by claiming he had assaulted them before dying of a synthetic marijuana overdose. The lawsuit noted that Harrell had no illicit drugs in his system when he died.

The lawsuit said the death was part of a pattern of brutality by rogue guards at the prison.