Los Angeles (CNN) The 911 call came in at 6:18 a.m.

A woman was screaming from somewhere inside a small apartment complex a block and a half from UCLA.

Officers from the LAPD's West Los Angeles Division hit their lights and sirens and sped to the scene.

By the time they arrived minutes later, the screaming had stopped.

The officers conducted a search and found no signs of trouble.

They left six minutes after they arrived without knocking on any of the doors of the eight-unit complex, including the one directly beneath the 911 caller who said that's where she thought the screams were coming from.

It was a decision that would cost two LAPD officers their jobs and, possibly, a 21-year-old UCLA student named Andrea DelVesco her life.

The following account is based on a CNN review of confidential police records, public court filings, coroner's records and other documents.

Together, they offer an unusual behind-the-scenes look at an unspeakable tragedy and raise questions about whether police did all they could to prevent it.

Andrea DelVesco was a person many others felt was their best friend.

An early morning disturbance

UCLA student Sarah Muhr got up early on the morning of September 21, 2015. Her boyfriend was visiting from Northern California to celebrate her 21st birthday and she needed to drive him to LAX to catch an early flight back home.

It was still dark when she returned to her second floor apartment on Roebling Avenue in the Westwood section of Los Angeles. The clock in her car read 5:30 a.m.

Muhr was at her front door about to turn the key when she heard a noise in the stairwell below. She turned to look and saw a man run away. She made a mental note of what he was wearing: Red and blue tank top, dark jeans and a baseball cap. Muhr was disturbed by the brief encounter but after talking with her boyfriend on the phone decided to go back to sleep.

She was awakened about 45 minutes later by a woman's screams and a barking, whimpering dog.

She called 911 and described the ruckus and the man she'd seen earlier to the operator.

"I know the girl below me has a dog," Muhr added.

As Muhr answered the operator's questions, a high-pitched noise that sounds like a woman screaming can be heard in the background, according to a transcription of the 911 call.

The operator asked Muhr if she could hear the sirens of the approaching police cars.

"Yeah, I hear them. They are getting close," she said. "OK, yeah they're here."

The operator then told Muhr: "I'm gonna make sure they get to the door below you first."

'No evidence of a crime'

Four LAPD officers riding in two patrol cars responded to the call.

Officers Rhoadell Sudduth and Thomas Montague were in one black and white; Alisha Williams and Erick Tillett in the other. Sudduth and Williams were seasoned veterans; Montague and Tillett their rookie trainees.

Williams and her partner interviewed Muhr, who repeated what she told the 911 operator, according to police records. The four then fanned out and conducted an exterior search of the complex, checking doors and windows for signs of forced entry.

Sudduth wrote that he watched as his partner shined a flashlight into the bedroom where DelVesco's body would be found on her bed about 30 minutes later.

"He observed the bed, but no occupants," Sudduth wrote.

His partner also shined his flashlight into the living room, "but observed no one."

At 6:30 a.m., seven minutes after they arrived, the officers cleared the scene.

"No evidence of a crime," Sudduth wrote.

'... her room is on fire'

Muhr remained shaken in her upstairs apartment. She kept looking out her window, but could no longer see the police.

"I was really nervous because they didn't come back and tell me if everything was OK," she would later say. "I just had this uneasy feeling."

She tried texting and calling some of the occupants of the unit below, each of them, like her, students and sorority sisters at UCLA.

She called aloud the nickname of one of the girls -- "Andy," short for Andrea, the girl with the dog. Andy was a free spirit known for her eccentric wardrobe, bubbly personality and compassionate heart. Many of her sorority sisters and friends considered Andy their "best friend."

Muhr heard nothing in response to shouting Andy's name.

She ran into her roommate, Erika, in the kitchen and told her about the screams and the police coming and going without letting her know everything was OK.

"She was like, 'don't worry. They probably did their job.' Like everything's totally fine," Muhr would later recall.

Moments later Muhr heard a loud bang from downstairs. She sprinted to the balcony outside her bedroom and looked down. She saw the same man she'd seen earlier that morning. He was jumping from the balcony outside Andy's room. She also saw smoke pouring from the apartment.

She called 911 for a second time just after 7 a.m.

"I just called the cops about a girl screaming and I just saw the guy who was in there run out of her room and it looks like her room is on fire," she said, according to a transcript of the call.

When the 911 operator asked for the race of the suspect, Muhr replied: "I already did this."

She went on to describe a young man wearing jeans, a tank top and baseball cap, matching the description she'd given earlier that morning.

"What color was the tank top?" the operator asked.

"We need an ambulance," Muhr responded.

In the apartment below, DelVesco's roomates, who'd gone to bed in the wee hours and slept through the screams, awoke to the sounds of a fire alarm.

Jessica Westling, who considered DelVesco "my best friend in the world," stood outside her friend's bedroom with another sorority sister, screaming Andy's name. They couldn't go inside to get her, Westling would later testify, because the room was already engulfed in flames.

The charred bed in Andrea DelVesco's room.

Firefighters found Andrea Lauren DelVesco's body on top of her bed.

Her face and fingers were burned so badly she was initially listed as a "Jane Doe" before being positively identified through dental records, according to police documents.

She suffered at least 19 "sharp force injuries," the coroner found. Some were superficial cuts; others stab wounds. Three-inch-deep wounds to both carotid arteries were deemed "rapidly fatal."

Her dog, a Chihuahua-terrier mix she called "Shay Panda," was found at the foot of her bed, singed and gasping for breath. The comatose animal was taken to a vet where it was euthanized to end its suffering.

Because DelVesco's body was so badly charred in the fire, investigators could not determine a precise time of death.

The absence of soot in her airways and relatively low level of carbon monoxide in her blood indicates she "was most likely deceased prior to ignition of the residential fire," her autopsy report states.

The blaze, an arson investigator would later testify, was intentionally set by someone who dumped a trashcan onto the bed and placed an open flame to its contents.

College students arrested

Alberto Medina, left, and Eric Marquez are former college students charged with killing Andrea DelVesco.

Two college students, Alberto Medina and Eric Marquez, were arrested and charged with DelVesco's murder. Medina, who attended Fresno State University, was visiting Marquez, who attended UCLA and lived less than a mile from DelVesco. The two, friends from high school, are accused of committing a burglary across the street from DelVesco's apartment. Prosecutors contend Medina then went inside DelVesco's unit and killed her. Marquez waited in the car and helped him cover up the crime, they allege.

Detectives recovered of pair of women's underwear they suspect was taken from DelVesco's body along with bloody clothing and a bloody knife in a closet in Medina's Fresno apartment, according to court records.

Investigators first identified Medina and Marquez as suspects after a pair of speakers allegedly stolen in the burglary were re-registered online by a man who turned out to be a friend of Medina's, according to police. The man told detectives he had done so at Medina's behest. Medina initially lied about how he came to possess the speakers, but eventually confessed to stealing them, according to police reports. He then confessed to being at the scene of the killing, but said his friend Marquez was responsible, the records state.

The slaying, with its grisly details and college student victim and alleged perpetrators, made headlines in Los Angeles and beyond. Medina and Marquez, both 23, have pleaded not guilty and are awaiting trial. Prosecutors have alleged special circumstances against Medina that would allow them to seek the death penalty if he is convicted. Attorneys for both defendants declined comment for this report.

On the morning of the killing, Muhr was interviewed by LAPD detectives who asked her if the officers who responded to the first 911 call knocked on the door of the apartment below her.

She told the detectives she saw one of the officers as she stood crying with friends after learning of DelVesco's death.

"He was like, 'I was the one that came earlier'," Muhr recalled.

And I was like, 'didn't you guys knock on her door?'"

"He said they didn't."

It remains unclear what transpired inside the apartment in the roughly 30-minute gap between the time the police left from the first 911 call and the time the second one was made.

Medina, the accused killer, was in the apartment from before the first screams were reported at 6:18 a.m. until he fled about 7a.m., according to the prosecution's theory of the case.

DelVesco's roommates were asleep in their rooms during that time.

Her body somehow ended up on top of her bed, which police had 30 minutes earlier found empty.

An investigator's note in the coroner's report suggests a chilling explanation for how she may have been missed by the officer's searching flashlight beam:

"Later examination of decedent's bedroom revealed 'a significant amount of blood' underneath her bed," the investigator wrote.

What really happened?

DelVesco's dog, Shay Panda, had to be euthanized because of injuries suffered in the fire.

Academically, Andy DelVesco was what one might expect of a student at UCLA: Nearly straight As in high school, National Honor Society, state spelling bee award and the like.

What set her apart, according to close friends and family, was her compassion. She was known to reach out to anyone who seemed troubled and had been volunteering since she was in the first grade. She spent her 16th birthday in Peru making bricks out of mud to help villagers construct shelters for guinea pigs, an important source of protein in the mountainous region.

"She loved helping people," her mother, Leslie, recalled. "That was just a huge part of her and her life."

Leslie DelVesco was among those who considered Andy a best friend.

The bond between the two remained strong even after she left the family home in Austin, Texas, to attend college at UCLA. Leslie DelVesco would get daily calls from her daughter from the campus sculpture garden, botanical garden, or other favorite spots.

She and her husband, Arthur, arrived in Los Angeles thinking that Andy had died in a fire.

But one of Andy's roommates told them she'd seen Andrea curled up in bed as she and others fled the apartment. She had blood on her shoulders and neck, the girl said, and did not respond to screams to get out.

"That's how we knew it was more than a fire that killed Andrea," Leslie DelVesco said.

She and her husband also learned that there had been a 911 call about a screaming woman that was made well before the blaze began.

Arthur DelVesco pressed for details about what had been done during the initial police response.

His questions were "very poorly received," he recalled.

"I was rebuffed vigorously," he said.

"They made him feel uncomfortable for even asking," his wife added.

Leslie DelVesco said she was astounded that the police left the apartment complex without knocking on the door of her daughter's apartment and making sure everyone inside was OK.

She noted that the officers arrived within minutes of the screams being reported and that Andy may still have been alive.

"We'll never know," she said. "I think the only person who knows is the person who killed her."

She said an earlier police response could have prevented another indignity: "We could have seen Andrea as we remembered her, before her body was burned so badly that they needed dental records to identify her," Leslie DelVesco said. "I could have held her one last time."

She said homicide detectives working her daughter's case persuaded her that it would be a mistake to view her body in the morgue. It's an image, one of them told her, "you can never erase from your mind."

Consequences for the officers

Disciplinary matters involving police officers in California are shrouded in secrecy. The details of individual officers' misconduct cases are shielded from public view, even when they result in firings.

When officers are faced with potential termination, they are directed to a so-called Board or Rights hearing, the LAPD's version of a trial. The hearings are held in the historic Bradbury Building in downtown Los Angeles and are closed to the public unless an accused officer elects to have them open.

Internal affairs officers present the case against the accused officers, who are typically represented by defense attorneys specializing in police misconduct cases.

Two LAPD command staff officers and a civilian representative listen to evidence for and against the accused then render a verdict and a recommended punishment.

It's the duty of the chief of police to actually impose discipline.

So the details of what happened with the internal affairs investigation are not publicly available.

CNN has confirmed through court records, documents, and interviews that Sudduth, a 21-year veteran of the department, and Williams, who had seven years on the job, were each found guilty at separate boards of rights hearings. Both boards recommend termination.

Chief Charlie Beck issued the order to fire Sudduth in May. The chief ordered Williams' termination in September. Neither firing received any publicity.

Both officers declined to speak with CNN through their attorney, Robert Rico. The outcome of the case was undeniably tragic, Rico said, but the officers were not to blame.

Rico pointed out that while Muhr thought the screams were coming from the unit below her, she also mentioned an echo effect in the complex's courtyard that made pinpointing the source of the sound difficult.

He called the information she conveyed in the 911 call "vague."

Rico, who was a police officer himself before he became an attorney, said the officers exercised proper "due diligence" in conducting the exterior search and doing so yielded no evidence of a crime.

In the officers' minds, Rico said, "There was no suspect; there was no victim."

He said the officers briefly discussed knocking on doors, but that Sudduth, the most senior among them, made a "command decision" and concluded there was insufficient information to begin waking up residents that early in the morning.

He said the officers' failure to knock on the door had no bearing on DelVesco's chance at survival.

"She was dead before the officers got there," he said.

He said Sudduth and Williams received overly harsh punishments not because of what they failed to do, but because of the subsequent tragedy they could not have anticipated.

"We don't fire people for not knocking on doors," he said.

Officers Tillett and Montague remain on the job. Both declined comment through their attorney, Gregory Yacoubian, who declined to say what, if any, discipline was imposed on the officers.

The DelVescos said they had been told nothing about the fate of the officers.

The couple stressed that they were grateful for the work of the LAPD homicide investigators in their daughter's case. But they said that didn't erase a lingering sense that they'd been stonewalled by the department with respect to certain aspects of the investigation and that there was a reason for why that had happened.

"They know they made a mistake," Leslie DelVesco said. "No one from the police department has ever apologized for that."

More than a year after Andrea's death, her father still finds it difficult to speak about her without becoming emotional.

"Countless times I've asked myself what I could have done to prevent this horrific tragedy," he said. "I can only hope that the LAPD asks themselves that question as well."

In response to an inquiry from CNN, LAPD spokesman Josh Rubenstein called the circumstances surrounding DelVesco's killing "a devastating and unspeakable tragedy."

"We cannot fully comprehend the grief her parents and other loved ones have suffered from their loss," Rubenstein said in a prepared statement.

He said the LAPD "took swift action to hold department personnel accountable for mistakes made in the initial stages of the investigation."

Rubenstein said he was barred by state employment law from providing further detail, other than to say "some of the officers are no longer employed by the LAPD."

"The LAPD will continue to work closely with the Los Angeles District Attorney's Office to ensure that those responsible for Andrea's murder are held accountable to the fullest extent of the law," the statement said.