LITTLE ROCK — The Arkansas Supreme Court ruled Thursday that the Arkansas State Police must release a criminal file that the agency claimed was exempt from disclosure because the investigation is ongoing — even though the case is now 54 years old.

The state’s highest court issued the ruling in a lawsuit filed by the Keech Law Firm of Little Rock after the Arkansas State Police denied a request the law firm submitted under the Arkansas Freedom of Information Act. The firm sought release of the agency’s file on the 1963 slaying of Ruby Lowery Stapleton in White County.

In a unanimous opinion Thursday, the Supreme Court affirmed a lower-court ruling that the file is subject to disclosure. Attorney General Leslie Rutledge had appealed the lower-court ruling, seeking to keep the decades-old file under wraps.

The Supreme Court said in its opinion that although files on ongoing criminal investigations are exempt from the Freedom of Information Act, law enforcement agencies are not allowed to invoke the exemption indefinitely. The court said allowing law enforcement agencies to claim the exemption indefinitely would insulate the agencies against legitimate probes by the public and the news media into the performance of their functions.

Although some activity in the case has occurred recently, at the time the law firm submitted its request there had been only “sparse activity” since 1965, and the recent activity has not made an arrest imminent, the Supreme Court said.

“This is a 54-year-old murder case. No charges have been brought or appear to be imminent. The victim’s family and the public are entitled to know how the officials in this case, i.e., law enforcement, performed their duties,” the court said in an opinion written by Justice Rhonda Wood.