On Friday, Alexander’s family filed a federal lawsuit in the District Court for the Eastern District of Pennsylvania against Delaware County, as well as the estates of the detectives and prosecutors on the case, calling their conduct “outrageous, malicious, wanton, willful, reckless and intentionally designed to inflict harm.”

“They murdered my brother. That’s what they did,” Susie Carter, Alexander’s 94-year-old sister, said in an interview on Monday. Ms. Carter, along with two of Alexander’s nieces, Osceola Carter and Osceola Perdue, are listed as plaintiffs in the lawsuit.

Delaware County officials did not immediately respond to a request for comment on Monday.

Ms. Robare’s body was found in her apartment, which was on the campus of the Glen Mills School, on Oct. 3, 1930. She had been stabbed 47 times with an ice pick. The body was discovered by her husband, Fred Robare. According to the suit, The Chester Times, a local newspaper, quoted Oliver Smith, a detective investigating the case, as saying that the murder had been committed by “a full-grown and strong man.”

An overturned lamp was the only sign of disruption. Ms. Robare’s keys were missing, according to the lawsuit, but a wristwatch and $15 had been left untouched on a bureau.

Investigators zeroed in on Alexander, one of the 600 students at the school, though it is unclear why. Alexander is described in the suit as having been 5 feet 5 inches tall and weighing 125 pounds, contradicting Mr. Smith’s description of the killer. Initially, Alexander denied having anything to do with the murder. But over the course of multiple interrogations, without a lawyer or his parents present — he eventually confessed. There was no physical evidence tying him to the scene. There was, however, a bloody handprint found at the scene, which did not belong to him.