French financial prosecutors on Tuesday raided several of France’s biggest banks, including Société Générale and BNP Paribas, as part of a multicountry investigation into what authorities say is one of Europe’s biggest tax thefts.

More than 150 financial investigators and 16 local magistrates swarmed the Paris-area headquarters of the French banks, as well as the offices of HSBC Holdings, Natixis and BNP’s Exane unit, in an early-morning action intended to gather evidence for an investigation of a tax avoidance scheme in which the banks allegedly bilked the French treasury out of vast sums.

The raids were part of what European authorities previously described as a developing investigation that spans four continents, dozens of banks and as many as 1,500 suspects. The banks under investigation in France were allegedly involved in a scheme known as “cum-cum trading,” from the Latin for “with-with,” in which individuals pocketed hundreds of millions of euros by avoiding the payment of French dividend taxes.

The French prosecutors’ office said in a statement that the banks raided on Tuesday had been using a strategy in which shareholders transferred stock for a short time to investors abroad to avoid paying the tax on the dividends and, in some cases, were able to get a tax refund. Investors then sold the shares back to the original owner, and the parties divvied up the savings. The government is looking to reclaim at least one billion euros, the prosecutors’ office said.

The French raids, which prosecutors said had been carefully prepared for months, also involved six prosecutors from Germany, where authorities have battled for nearly a decade to bring a ring of financial traders and bankers to justice for bilking the government out of billions of euros in tax revenue through a similar fraud.

The scheme in Germany was built around “cum-ex trading,” Latin for “with-without,” a monetary maneuver that savvy investors used to produce two refunds for dividend tax paid on one basket of stocks.

Overall, several European countries are seeking justice in what the French daily Le Monde, which first reported the French scheme in 2018, has called “the robbery of the century.” For years, hundreds of bankers, lawyers and investors were able to siphon an estimated $55 billion from the state coffers of European countries through the schemes.

All told, Germany has been hardest hit, with an estimated $30 billion in losses, followed by France, which has lost an estimated $17 billion. Smaller sums were drained away from Spain, Italy, Belgium, Austria, Norway, Finland, Poland and others.

Many countries in Europe were targeted by cum-ex traders, with much of the activity starting in the early 2000s and rising in popularity in the wake of the Great Recession, when much of the financial industry was reeling. In recent years, countries had closed the loophole that allowed cum-ex trades but often failed to notify the tax authorities of their neighbors.

In December, a lawyer described by the authorities as the brains behind the tax scheme in Germany was sentenced to eight years in prison by a court in Bonn. The lawyer, Hanno Berger, once worked for the German government and had earned a reputation as one of the country’s most fearsome tax inspectors. He later went into private practice and eventually became a ringleader in cum-ex trading, German prosecutors said.

The scheme has also led to civil and criminal cases in Britain as well as Denmark, where officials say the country’s tax agency was swindled out of €2 billion by a London-born financier who moved to Dubai.

The French prosecutors urged anyone wishing to bring further information related to the French inquiry to come forward.