He was living in apartment with Mann but nature of relationship is unclear

They want to find Gambian who had lost right to stay in Austria

An American nanny has been found 'murdered' in her apartment in Vienna, local police have confirmed.

Lauren Mann was discovered lying half-naked, face-down on a mattress at her fourth district address on Tuesday.

Authorities say the 25-year-old's body was found surrounded by a 'considerable amount of blood' as police confirmed they are treating her death as a homicide.

Lauren Mann (pictured in a Facebook photo) was discovered lying half-naked, face-down on a mattress at her fourth district address on Tuesday

She had a pullover across her face and her arms were stretched along her body, local reports said.

A Gambian man who had lost his right to stay in Austria had been living with her in the apartment but has not been traced.

He is one of the focuses of the police inquiry, a source told one local newspaper. The nature of their relationship was not explained.

Mann, a nanny from Paonia, Colorado, had been studying and working in Vienna for around three years.

According to The Local, the young woman's employer became alarmed when she failed to arrive for work on Monday.

He called police who, with the help of firemen, broke open the door to her studio apartment on the Wiedner Hauptstrasse on Tuesday evening.

They found her body inside in complete darkness. All of the light bulbs in her apartment had burned out.

Authorities say the 25-year-old's body was found surrounded by a 'considerable amount of blood' as police confirmed they are treating her death as a homicide

Mann, a nanny from Paonia, Colorado (pictured in Facebook photos) had been studying and working in Vienna for around three years

An autopsy showed she had no external injuries. Detectives have now ordered a toxicological report to check if she had any suspicious substance in her bloodstream.

Police spokesman Thomas Keiblinger said blood found near her head may indicate an internal injury.

Her employer described her as 'very reliable', hence why he found it unusual when she did not arrive at work.

Mann had studied piano and French at the University of Colorado before moving to Europe to study German in Vienna.

Her friends have begun paying tribute to the 25-year-old after she was found dead in her Vienna apartment yesterday.

Speaking exclusively to Daily Mail Online, close friend Martin Škubal described her as 'a sensitive and nice girl' and added that he 'loved to speak with her about music'.

Škubal, who also lives in the Austrian capital, continued: 'I'm really sad about it [Lauren's death].

'Lauren was a sensitive and nice girl and I loved speaking with her.'

Another close friend, Svante Helmbæk Tirén, took to social media to share the news of her death and wrote on Facebook that he will 'miss her every day'.

The Swede, who studied at Vienna's Akadmie der Bildenden Künste Wien, added: 'Today I have lost one of my dearest friends, Lauren.

'I will miss her every day and my heart goes out to all her family and friends. Please remember to cherish and be grateful for all the wonderful friends and people in your lives.'

The creative 25-year-old described herself as an avid reader and lover of art, and literature and music.

The U.S. Embassy in Vienna confirmed her death and said it was working with Austrian authorities on their investigation.

'We extend our deepest condolences to her loved ones,' the embassy told NBC News.