Okay, okay..."probably" might be a bit of an exaggeration, but it was one of the top theories. In Sweden, it is known as the "Atlas Vampire Murder", named for the neighborhood in Stockholm where it took place in 1932. Even today, over 50 years later, the case is still as cold, and enthralling, as it was in the 1930s.
Lily Lindstöm was the victim of this killer. She was a thirty-two year old prostitute that often entertained guests in the comfort (and likely safety) of her own home. While, at first, it may seem strange that she invited men up to her apartment, it does make a little sense -- home field advantage. She might have even had friends or neighbors in the building that would have helped her, should one of her johns make a bad move.
In fact, one of her friends and downstairs neighbor, was the last to see her alive. Minnie Jansson was also a sex-worker and saw her just days before her body was found. According to Minnie, Lily had knocked on her dorm to get some condoms. When her friend didn't visit the next day, Minnie began to get nervous as the two talked quite often. She called the Stockholm police and they made a visit to Lily's apartment a few days later.
On May 4th they entered her apartment and the Stockholm police beheld a horrific scene. To avoid being too graphic, you can read all the grisly details here. But, in summary - Lily was face-down on her bed. She was not wearing any clothes, and, instead, they were neatly folded on a chair near her bed. According to the police, it appeared she had been dead for roughly 2-3 days. There was also proof she had sexual contact shortly before death. The cause of death were repeated blows from a blunt object to her head.
I'm sure you are wondering, a little, where exactly the "vampire" part of this unsolved crime comes into play. Well, in addition to the horrific way Lily met her end, it was determined that most (if not all) of Lily's blood had been completely drained. Furthermore, saliva was found on her neck. And, after further investigation of the crime scene, a blood-stained gravy ladle may have been used to consume her blood.
Thus this haunting murder was more than just a tragedy - it may have been committed by a true monster (or, at the very least, a seriously deranged person who believed they were a monster.)
No one was ever charged for death. It was believed her last customer was the perpetrator, but after interviewing many of her regulars the police came up empty. And they also had another lingering question - where did all her blood go? Because it wasn't in the apartment.
Over the years, many different theories have come forth. Some are more realistic than others. For example, some believed it was a police officer who was able to throw the others off his tracks by creating an elborately weird crime scene. However, the little evidence found does not seem to wholly support this theory.
This remains one of the creepiest, and truly most bizarre unsolved cases, that Stockholm has ever seen. It continues to grip many people to this day and you can even see some of the evidence, which remains on display. You can see the picture here.
The above picture is from the early Gothic vampire novel, "Carmilla". It is liscensed under public domain and is unrelated to the above story.