A painting by the Austrian artist Gustav Klimt that was considered lost for 100 years has sold for €30m (£26m) at an auction in Vienna.

Entitled Portrait of Fräulein Lieser, the unfinished picture was painted in the spring of 1917, when Klimt was one of the most celebrated portraitists in Europe, and a year before his death.

Until the Viennese auction house im Kinsky announced at a press conference in January that the portrait had been re-discovered in a private collection, only a black-and-white photograph of the painting was known.

The artwork’s sudden re-emergence, coupled with an intriguing backstory, built up considerable buzz around the painting.

In the run-up to Wednesday afternoon’s sale, about 15,000 visitors had taken the opportunity to see the work displayed at im Kinsky.

The sum the painting fetched was at the bottom of its €30m to €50m valuation, and some way off the £74m that another late-period Klimt portrait, Dame mit Fächer (Lady with a Fan), fetched in London last June – a record for any artwork ever sold at an auction in Europe.

Some key questions about the painting remain unanswered, including the identity of its subject and its provenance during the Nazi era.

A commissioned portrait, it is believed to depict one of the daughters of either Adolf or Justus Lieser, who were brothers from a wealthy family of Jewish industrialists. Some art historians have identified the sitter as Margarethe Constance Lieser, Adolf Lieser’s daughter.

However, im Kinsky has suggested the painting could also depict one of the two daughters of Justus Lieser and his wife, Henriette, a patron of modern art. An AI-based “ageing” of the portrait shows up apparent similarities to Helene Lieser, an economist who died in 1962, the auction house has claimed.

The identity of the sitter is crucial: after Klimt’s death in 1918 the painting ended up with the Lieser family, though which branch of the dynasty could rightfully claim ownership of the picture has been hard to establish. Art historians have struggled to piece together the painting’s journey between 1925 and 1961.

The auction house said the painting’s journey during the Nazi period was “unclear”.

It said: “What is known is that it was acquired by a legal predecessor of the consignor in the 1960s and went to the current owner through three successive inheritances.”

The identity of the last Austrian owners before Wednesday’s sale has not been made public.