Authorities in Colorado are investigating the murder of an Evergreen woman who they say was killed by her younger brother. A motive in the case remains unclear, but the details surrounding her death are bizarre.

A Jefferson County Sheriff's Office arrest affidavit indicates that deputies were dispatched to the 30000 block of Hilltop Drive on April 8 in response to a report of a female who was not breathing. Upon arrival at the scene, deputies were greeted by 17-year-old Rebecca Santaguida who directed them to the bottom of a basement staircase, where her sister, Elizabeth Santaguida, 24, lay sprawled on the floor.

"Medical personnel responded, and it was determined that the female was deceased," Sheriff's Office Public Information Officer Mark Techmeyer said in a press release.

Rigor mortis had already set in, suggesting that Elizabeth had been dead for several hours. Initial observations of her body showed a ligature mark around her neck; however, investigators were unable to determine what caused the injury. The affidavit also states that Elizabeth's pants were pulled down, exposing her panties.

When questioned by police, Rebecca said that she had called her father, Richard Santaguida, at about 10:30 p.m. to find out when her sister would be home. Richard is a doctor who evaluates patients to determine whether they qualify for medical marijuana. Elizabeth worked with her father, assisting him in his work.

Rebecca said that when her father indicated Elizabeth had not shown up for work that day, she went downstairs, and made the shocking discovery.

Richard arrived on the scene shortly after the police. He said he had last seen Elizabeth when he left for work, at about 12:45 p.m., and that she had agreed to meet him later that day. Elizabeth never showed up and when he called her at 5 p.m., she did not answer.

During Richard's conversation with police, he asked to see his daughter's body. Deputies denied the request. According to police, at that point he yelled at his wife, saying, "Why did you call the cops? You should have waited until I got home."

Police say that Richard "reluctantly" gave consent for a search of the home. He informed them that they would find roughly two ounces of marijuana for which he had a license, but, when they asked to see the license, he said that it was in the basement with his daughter's body.

During investigator conversations with the family, Elizabeth's 18-year-old brother, Zachariah, sat at a table cutting his fingernails with nail clippers. He told police that he had been home the entire day, but had slept through most it. He said he considered himself a "shut in."

When investigators searched the Santaguida home, which they described to be "in complete disarray," with "clutter and trash" scattered throughout, they found numerous prescription medications, beer cans and a Jagermeister bottle in the basement.

Richard told police that Elizabeth was on numerous "psych" meds, but that she was not capable of hurting herself.

By 6:30 a.m. Friday, each of the family members was at the Jefferson County Sheriff's Office for another round of questioning.

Zachariah admitted to police that he had gone downstairs on the afternoon of his sister's death to retrieve some "DVDs" and "beef raviolis." He said that, while he was down there, his sister had surprised him and told him to put the items down, so he did. He also admitted to watching pornographic films and said that, on occasion, he had taken those types of movies from his sister.

The second interview with Rebecca was more revealing.

She said that she believed her brother had recently stolen a bottle of Jagermeister from her sister. When she told Elizabeth of her suspicion, she said that her sister noticed that a vibrator she had stored next to the bottle was also missing. Elizabeth told her that the sex toy, which she had received as a "gag gift," was unused and still in the original packaging. The vibrator was later found hidden beneath the cushions in a couch. The affidavit states that it was not in the original packaging.

Rebecca told police that she did not know whether Elizabeth had confronted Zachariah about the stolen items.

Much of Zachariah's interview that is found in the arrest affidavit has been blacked out by prosecutors. It concludes that, based upon the foregoing facts, "there is probable cause" to take him into custody on suspicion of murder.

The Jefferson County Sheriff's Office will not comment on the reasons behind Zachariah's arrest, and neither will the district attorney's office.

"We are prohibited from talking about it, but certainly there would have been evidence at the scene that led them to believe they had the probable cause to make this arrest," Jefferson County district attorney's spokeswoman Pam Russell told AOL News, adding, "We redact any of those things from affidavits, and that would include if he had made any statements incriminating himself."

Russell said that a cause of death for Elizabeth has not been released and that the coroner has not yet revealed whether she had been sexually assaulted.

"There hasn't been any information released from the coroner's office, and there probably won't be any of that information available until a preliminary hearing," Russell said.

Russell says that Zachariah had not been in trouble with the law prior to his arrest. "There is none here -- no criminal history," she said.

According to cbs4denver.com, Zachariah recently dropped out of Evergreen High School, where he was a senior. Former classmates have given mixed opinions of him. Some say he was a loner, whereas others describe him as jubilant.

"Whenever I like, see him, he seems to be all fun and joy and stuff like that. But when he's with other kids, he seems to be out of control," a classmate, Josh Rivera, told the Web site.

On Monday, Zachariah appeared in a Jefferson County court, where he was advised that he is being held on suspicion of first-degree murder. Bail was not set. He is expected to be formally charged today.

"My office has until tomorrow morning to get the charges filed, so we will take the evidence that the sheriff's office has put together during their investigation, apply the law, and determine what we think are appropriate, if any charges," Russell said.