The killing of law professor Mary Joe Frug on a quiet street in Cambridge had all the makings of a paperback mystery. Maybe too many.

After the brutal 1991 slaying, police spent considerable time chasing down a far-fetched theory that seemed straight out of “Murder, She Wrote” — that Frug, a respected feminist scholar, had been stabbed to death by an academic rival.

Some of the evidence did seem to point to a targeted killing: an unknown assailant stabbed Frug on Sparks Street and then vanished. Detectives said the killer had been full of rage, based on the vicious nature of the attack. The murderer didn’t take her purse, seemingly ruling out robbery as a motive. And Frug’s feminist scholarship, supposed some observers, seemed controversial enough to make enemies.

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But according to Globe reports at the time, her colleagues scoffed at that idea.

“We interviewed people who disagreed with her politics. But people say, ‘Look, when I disagree with someone, I write an article saying how stupid they are and what I believe is right. I don’t go out and kill them,’ ” a State Police investigator said in a Globe article marking the first anniversary of the killing.

Investigators eventually moved away from the theory that Frug, a professor at the New England School of Law and the wife of Harvard Law professor Gerald Frug, had been killed by a rival or a student angry over a grade. Indeed, they were never able to determine whether Frug had been targeted at all or was the victim of a random attack.

At the first anniversary of Frug’s death, the New England School of Law put up a $25,000 reward for information leading to an arrest, hoping to spur what was already a slowing investigation.

But since then, the trail has only grown colder.

A spokeswoman for Middlesex District Attorney Marian Ryan said investigators were still actively investigating the case, and urged those “with any information that might be helpful, no matter how seemingly insignificant, to contact law enforcement.”

“The Middlesex district attorney’s office has continued to examine evidence and follow up on people described by witnesses who saw things in the area before and after the murder,” she added. “We also continue to look into factors personal to Ms. Frug and her family. Although to date we have been unable to identify who killed Frug or what the motive may have been, we continue to try to answer those questions. . . . The investigation remains open and ongoing.”

Frug, meanwhile, hasn’t been forgotten. Her work was celebrated in a recent special edition of New England Law Review published on the 25th anniversary of her death.

“After such a horrifying event there is no such thing as ‘closure,’” her husband wrote in the edition’s introduction. “The murder left a permanent wound.”

Produced by Elaina Natario, Laura Colarusso, Heather Hopp-Bruce, Alex Kingsbury, Jeremy D. Goodwin, and Mary Creane

Lead image by Alejo Porras