Hervé Falciani, a 40-year-old French-Italian citizen, was arrested in Barcelona in July 2012 after he arrived by boat from the port of Sète in France.
He was apprehended under an international warrant seeking his extradition to Switzerland, where he is wanted for violating banking secrecy laws.
He collected data on at least 24,000 customers of HSBC's Swiss subsidiaries from 2006 to 2008 while he worked in the bank's information technology development unit in Geneva.
He then passed on the information to French authorities.
The files, which were subsequently relayed by French investigators to their counterparts in the United States, Spain, Italy and several other European Union countries, led to a raft of prosecutions.
Falciano told daily newspaper El Pais that about a month before he fled to Spain, US justice officials who he was collaborating with from Paris warned him that his life was at risk.
"The United States warned me that it would be easy for someone to pay to try to kill me," he said, according to a report published on Sunday.
"I had to plan my escape carefully," Falciano said.
"I chose Spain knowing that I would go to jail and that Switzerland would ask for my extradition," he said.
"I had two options: start a new life in the United States or travel somewhere else to gain time," Falciano said.
"They told me that the only safe place in Europe would be Spain, which had used my information with success in important cases.
"They thought it would be very unlikely that Spain would approve my extradition to Switzerland."
Spanish prosecutors opposed Falciani's extradition to Switzerland during his court hearing on April 15th in Madrid.
The court is expected to give its decision in the coming weeks.
Falciani told the court in Madrid last week his intention was to raise the alarm about what was going on at the bank and denied he sought to sell the information as alleged by Swiss officials.
In December, Spain's High Court ordered Falciani's release from jail pending his extradition hearing after the Spanish authorities argued Falciani was cooperating with authorities in several European countries in investigations into tax evasion, money-laundering, corruption and terrorism financing.