(NewsNation) — There’s a general rule of thumb when it comes to homicide investigations: the longer a case goes without an arrest, the less likely it is to be solved.

This is often thought of as the “48-hour rule” or “The First 48,” as the popular A&E show is named. It’s based on the idea that the first two days are the most crucial in an investigation.

“After the first two days, competing resources, competing interests, perishable evidence, alibis — all of these things become increasingly problematic,” said Michael Arntfield, a criminologist and co-director of the Murder Accountability Project, a nonprofit that focuses on unsolved murders.

The 48-hour adage will come as no comfort to those in Moscow, Idaho, where nearly three weeks ago four University of Idaho students were found stabbed to death in an off-campus home.

Authorities have yet to identify a suspect or find a murder weapon in the case, but that doesn’t mean it’s gone cold.

Although data can be difficult to trace, multiple analyses have found that most homicide investigations don’t lead to an arrest in the first 48 hours.

A Washington Post report from 2018 examined 8,000 homicide arrests across 25 major U.S. cities and discovered that only 30% of cases had an arrest in the first two days. About two-thirds of arrests were made within one month after the incident.

But for cases that remained unsolved after one year, just 5% ultimately led to an arrest, The Post determined.

A separate 2021 analysis in the New York Times found that the amount of time it takes to clear a murder varies widely from city to city.

Just two of the 13 police agencies studied cleared more than half of their murder cases within the first 48 hours, the Times found. At the higher end, police in Louisville, Kentucky, solved 62% of homicides in the first 48 hours, whereas authorities in St. Louis, Baltimore and Chicago cleared murder cases in the first two days less than 25% of the time.

Much of that discrepancy can be explained by the sheer number of homicides and thus the amount of resources available for each case.

“The more resources, the more detectives that are available to work a case — be it a murder or a non-fatal shooting — the more likely you are to find that really important evidence,” said Jeff Asher, the co-founder of AH Datalytics who authored the New York Times report.

With local, state and federal law enforcement investigating, the importance of significant resources bodes well for a case like the University of Idaho killings.

There are other factors working in law enforcement’s favor: the size of the town and the manner of the killings.

In a small community like Moscow, Arntfield says the pool of potential suspects is more stable than a large urban area.

And both experts agreed stabbings, as opposed to gun-related homicides, are more likely to result in DNA evidence — either on a murder weapon or left by the perpetrator at the scene — that investigators can use to identify a suspect.

But cases near college campuses also present unique challenges: social circles are vast and everchanging, students’ schedules tend to be more irregular than someone working a 9-5 job and living situations can be fluid.

“You’ve got four different people who were socially active, engaging with dozens, if not hundreds of people virtually,” said Arntfield. “Their routine becomes very difficult for police to nail down.”

Arntfield said he suspects there is physical evidence at the scene but that the offender may not be in the DNA databank, which would mean investigators have nothing to compare it to. He said that’s more common than many people realize.

On Wednesday, hundreds of community members gathered for a vigil to honor the victims: Ethan Chapin, 20; Madison Mogen, 21; Xana Kernodle, 20; and Kaylee Goncalves, 21.

At the vigil, Steve Goncalves, Kaylee’s father, disclosed that his daughter and Mogen were killed together in the same bed. He said Kaylee and Madison were lifelong best friends who were together until the end.

NewsNation has been covering this story from the beginning, and will continue its investigation in a special hour on Sunday. National correspondent Brian Entin will host “Special Report: Idaho Murder Mystery” on Dec. 4 at 9 p.m. ET on NewsNation. Here’s how you can watch.