SIOUX CITY – Donald E. O’Brien, a longtime federal judge in Iowa known for landmark rulings that improved jail and prison conditions for inmates, has died at age 91.

O’Brien died Tuesday at his home in Sioux City after an illness, according to a statement released by the federal court in the Northern District of Iowa. O’Brien had continued working a reduced caseload up until his death.

“He was an exceptional public servant and extraordinary federal district court judge known far and wide for his unsurpassed compassion, fairness, and commitment to doing justice,” the statement said.

By the time President Jimmy Carter appointed O’Brien to the federal bench in Iowa in 1978, he already had a distinguished career in government service. A decorated World War II veteran, O’Brien had served six years as the top federal prosecutor in northern Iowa after a 1961 appointment by President John F. Kennedy.

His 37-year career as a federal judge included many high-profile cases. One of O’Brien’s most significant rulings came in 1997, when he declared the conditions at the Iowa State Penitentiary in Fort Madison to be shameful.

Siding with inmates who had filed a class-action lawsuit challenging the conditions as unconstitutional, O’Brien found inadequate mental health treatment that caused all inmates to suffer from out-of-control behavior and extraordinarily long lockup sentences for misbehaving inmates that deprived them exercise during winter months. The ruling angered the Iowa Department of Corrections and Republican Gov. Terry Branstad, who blasted O’Brien as a liberal who cared more about inmates than the public.

Undeterred, O’Brien prodded state officials in the coming years to draw up detailed corrective plans that ultimately led the prison to build a new 200-bed facility that improved care for mentally ill inmates.

In 1987, O’Brien had sided with a class of juvenile offenders by ruling that Iowa authorities were violating federal law by continuing to jail too many juveniles in adult jails. He ordered the state to draw up plans to reduce the rate and comply with the Juvenile Justice Act, which led to juvenile jails being built around the state that still house youthful offenders today.

“He worked tirelessly so that prisoners were treated humanely and fairly, and considered his work on behalf of prisoners his greatest accomplishment,” the court’s statement said.