The email demanded $2,800 (£1,975) in bitcoin and threatened, “I will be short. I’ve got an order to kill you”.
Christine, who chose not to give her surname, said: “I knew no one was tracking me.
“But I found myself as I was on my way to work looking around.
“Are any cars following me? Does anyone look suspicious?”
FBI agent Laura Eimiller said it is a new spin on extortion.
Agent Eimiller said: “The chances are if you are online, you will be victimised not once, not twice, but multiple times.”
Investigators said the emails are carefully designed so that even educated professionals can be lured in.
Christine went on the FBI’s Internet Crime Complaint Centre which gives safety information and tracks threatening activity.
Agent Eimiller added: “We receive an average of 800 complaints a day in the United States on that site.
“We believe it represents about 15 per cent of the scams that are actually taking place, so it is heavily underreported.
“If only one per cent of people send money to them, there’s no overhead for them. That is money in the bank.”
Bitcoin transactions provide anonymity to users and so has been used for criminal activity such as buying illicit items on the dark web.
It also provides anonymity for scammers as the virtual currency does not pass through any banking institution and consumers cannot stop payment like they can with a credit card.
Cyberfraud, drug dealing, prostitution, gun-running and other major crime profits are being ploughed into the internet currencies.
Drug pedlars are using high street bitcoin ATM machines to deposit cash from deals, and there are 77 such ATMs in Britain.
Christine said she was disturbed even though she knew it was a scam
Head of Scotland Yard’s Serious and Organised Crime Command Detective Chief Superintendent Mick Gallagher, said gangs have turned to cryptocurrencies.
He said: “At the moment, it feels like there is significant growth.”
Online criminals prefer the added privacy of some of bitcoin’s competitors as forensic firm Chainalysis said the amount of bitcoin being used on the Dark Web has fallen from 30 per cent to one per cent.
Cybercriminals are turning to other digital currencies
Philip Gradwell, chief economist at Chainalysis said: “In the last few months, there has been a rapid increase in the use of Monero, likely for illicit means.
“Whether Monero grows further, to displace bitcoin as the crypto-crime currency, depends on its adoption by new darknet markets, which are emerging following recent shutdowns, and improvements in the user experience of buying and transacting in Monero.”