The FBI has put 42-year-old Bulgarian woman Ruja Ignatova, also known as the ' Missing Cryptoqueen' on its Ten Most Wanted Fugitives list for cheating investors of more than $4 billion in a cryptocurrency scam.

The FBI has offered a $100,00 reward to anyone who can provide useful information to track down Ignatova, who the Department of Justice said used her OneCoin cryptocurrency as a ponzi scam to target investors worldwide between 2014 and 2018.

She is the only woman on the FBI's 10 most wanted list.

In 2014, OneCoin, a self-described cryptocurrency, began offering buyers commission if they sold the currency on to more people.

But FBI agents say OneCoin was worthless and was never safeguarded by the blockchain technology used by other cryptocurrencies.

According to allegations made by federal prosecutors, it was essentially a Ponzi scheme disguised as a cryptocurrency.

"She timed her scheme perfectly, capitalising on the frenzied speculation of the early days of cryptocurrency," BBC cited Damian Williams, Manhattan's top federal prosecutor as saying.

Investigators believe Ignatova was tipped off in 2017 after a U.S. District Court in New York issued a warrant for her arrest, which enabled her to escape to Greece and disappear.

Special Agent Ronald Shimko, who is investigating the case out of the FBI's New York Field Office, said he hopes the publicity of being on the most wanted list will lead to the arrest of Ignatova.

'There are so many victims all over the world who were financially devastated by this,' Shimko said. 'We want to bring her to justice,' The Daily Mail quoted him as saying.

"We also believe she has high quality fake identity documents and has changed her appearance," he added.