A moment of silence during a vigil at the University of Idaho on Wednesday.Credit…Rajah Bose for The New York Times

MOSCOW, Idaho — Every few hours comes another call. A food delivery driver who heard a woman screaming. A mother asking the police to walk her daughter to her car after work. A woman who woke up at 3 a.m. to find her front door wide open.

The flood of calls to the Moscow Police Department is a sign of just how afraid people in this college town have become, three weeks after four University of Idaho students were fatally stabbed by an unknown assailant in their bedrooms in the middle of the night.

Many students refused to come back to campus after Thanksgiving, and some classrooms at the university now sit half empty. Those who did return said they bought doorbell cameras, put rods in their windows to lock them shut or began hunkering down with roommates at night.

“I ask my friends for rides all the time,” said Jemimah Tudi, a sophomore from India who said she no longer walks alone after dark and expects to get pepper spray for Christmas.

The fear that sits over this city of 25,000 people in the rolling hills of northern Idaho is unlikely to ease until the killer or killers are caught. But there is little indication that the police are any closer to making an arrest than they were on the day of the killings, Nov. 13, when news of the stabbings sickened residents and turned this normally idyllic college town into the scene of a national mystery.

The police have issued sometimes contradictory statements, leading at least one victim’s family to question whether investigators are up to the task of solving a quadruple homicide in a city that had not seen a murder since 2015.

“So frustrating. No info at all,” Alivea Goncalves, the older sister of one victim, Kaylee Goncalves, said in a text message last week after the police held their most recent news conference. “They have done nothing to gain any of our trust.”

The number of F.B.I. and Idaho State Police investigators working on the case — including behavioral analysts trained to outline a possible profile of the killer — now vastly outnumbers the 36 total employees in the Moscow Police Department.

“We may not have identified a suspect yet, but we are getting a clearer picture of what happened,” said Aaron Snell, a spokesman for the State Police.

The house at 1122 King Road is tucked away on a dead-end street about a five-minute walk from the fraternity houses that line one edge of campus, with cars packed tightly into driveways and students often walking to and from class along snowy pathways.

The three-story house was a place where friends often socialized and posed for smiling pictures, social media posts from earlier this year show. But for the past three weeks, it has sat empty, marked off by police tape and guarded day and night by a police officer.

The slain students’ possessions remain behind: a pair of pink cowboy boots just inside a third-floor window, a neon sign on a wall that reads “good vibes,” a couch collecting snow on the back patio.

Killed in the early-morning attack were Kaylee Goncalves, 21, who was planning to graduate in the winter and move to Austin, Texas; Madison Mogen, 21, who loved concerts and had worked from a young age to help support herself; Xana Kernodle, 20, a marketing major who had begun to blossom while living away from home; and Ethan Chapin, 20, Ms. Kernodle’s boyfriend and a triplet who seemed to be always smiling or telling a joke. The three women lived in the home, and Mr. Chapin was visiting his girlfriend.

On Saturday, Nov. 12, Mr. Chapin and Ms. Kernodle spent the evening at a fraternity party while Ms. Goncalves and Ms. Mogen went to a sports bar in town. They all returned shortly before 2 a.m., and phone records show that a series of calls were soon placed from Ms. Goncalves’s phone to her former longtime boyfriend, Jack DuCoeur, who is also a student at the university, her older sister said.

Mr. DuCoeur did not pick up, and there were six more calls until 2:52 a.m., when they stopped. Several calls to the same number were also placed at about the same time using Ms. Mogen’s phone, the police said.

Little is known about what happened after that.

What the authorities have said is that, at some point in the night, someone armed with a large knife attacked the victims, likely as they slept, and managed to escape without waking the two additional roommates. Ms. Goncalves’s father said that Ms. Goncalves and Ms. Mogen were in the same bed when they were killed.

It was not until just before noon that the two surviving roommates realized something was wrong. The police said that they first called friends over to the apartment, believing that one of their roommates was passed out, and that someone called 911 shortly after.

When the police arrived, they found a gruesome scene, but no murder weapon or sign of forced entry. No possible motive has been disclosed and there are no suspects. The police said they had learned through interviews that Ms. Goncalves may have told friends she was worried about a stalker, but the authorities have not been able to verify that.

The shock of neighbors and students quickly gave way to fear. Residents began checking their locks, texting one another their whereabouts and calling the police over everything that seemed out of place — a revved engine in a Walmart parking lot, a man seen “wandering around.”

Neighbors also began sharing the story of a man on the outskirts of town who reported that his neighbor’s dog had been found a few weeks before the murders, skinned from neck to legs. (The police reassured the public that they “do not believe there is any evidence” that the incident was related to the students’ deaths.)

Seeking to calm the community, the police quickly said they believed there was no “ongoing community risk” or “imminent threat.” An initial statement from the police that the attacks were “targeted” was walked back and forth, with Bill Thompson, the Latah County prosecutor, saying at one point that he had no more information than the public about why the police had called it that.

“That’s what they told us and we accepted that at face value,” he said.

The claims never made sense to locals, students or their parents, since the police were also saying they did not know who had committed the killings, or where they might be. Chief James Fry of the Moscow Police Department ultimately conceded, three days after the crimes, that the police “cannot say that there is no threat.”

The back and forth has done little to calm residents like Angelica Silva, who said her husband had come home last week to find one of their windows wide open. What might have been considered odd in normal times was instead “super unsettling,” Ms. Silva said, with their young daughter at home.

“The curtains were hanging out the window,” said Ms. Silva, who has lived in Moscow since she was a child. “We are definitely triple-checking everything now.”

As the case drags on, there are worries that the investigation could go cold, leaving the town in a state of paralysis. But Chief Fry dismissed that idea this week.

“We’re going to solve this,” he told The Moscow-Pullman Daily News. “We’re going to continue to work until we solve it.”

Blaine Eckles, the dean of students at the university, said about a third of students who live in residence halls had not returned, though he did not have a figure for how many of the vast majority of students who live off campus have switched to online learning. As some students returned to campus on Monday following the holiday break, they said their classes were emptier than usual, and a heaviness could be felt over campus.

“It’s still so unknown and you don’t know what’s going on,” said Helen C., a senior who declined to give her full last name because she feared for her safety. “I’m hopeful, but it also seems like the further you get away from it, the easier it is to not find anyone, and then — not to be scary — you start thinking about Ted Bundy and all the stuff he did.”

Helen said she and her roommate had recently invited a friend to spend the night with them after learning that the friend’s roommates had not yet returned to town, leaving her alone at home.

Some students said that even though they worried about the possibility of another attack, they did not like the thought of returning to remote classes after doing so for long stretches during the coronavirus pandemic.

“Being an engineering student, I didn’t really have a choice,” Jaydon Morgan, a freshman, said after attending a noticeably empty calculus class. “It’s either in-person or struggling at home.”

At a vigil on the football field of the Kibbie Dome this week, students wiped their eyes as relatives of the victims spoke of their grief.

Ms. Mogen’s father, Ben Mogen, said he had always been proud to brag about his daughter to friends or people he was meeting for the first time, rattling off her academic accomplishments and pulling out pictures.

“I just would tell them all about Maddie,” he said.

All of the victims were members of fraternities and sororities, and many of those in attendance were in the university’s Greek organizations.

Chris Bofenkamp, a University of Idaho graduate, attended the vigil and said the killings had hit her especially hard because she had been a member of the same sorority as Ms. Kernodle and Ms. Mogen.

“That hit home for me because I remember how tight it was with the people I lived with,” Ms. Bofenkamp said. “You’re away from home, they become your family, and when they say, ‘It’s your sisters,’ it really feels like that.”

Mike Baker and Serge F. Kovaleski contributed reporting. Sheelagh McNeill and Kitty Bennett contributed research.