The Swiss authorities made a show on 18 February of raiding the offices of HSBC's private bank in Geneva over suspicions of money laundering - they even took the unusual step of informing the media about the raids.
The prosecutor's office said in a statement that it is opening "a criminal procedure against the bank and unknown others for aggravated money laundering".
The documents revealed by whistle-blower Hervé Falciani are said to contain lurid details of accounts at the banks connected to drug dealers and known criminals, but stolen evidence would not be admissible in a Swiss court, so they are going on the hunt themselves.
However, helping clients evade tax - which so far seems to be the only thing to have definitely happened - can't be the foundation of any Swiss case against the British bank.
Currently Swiss money laundering rules do not include tax evasion as a predicate offence to money laundering. Switzerland plans to amend its law so that it will, according to the Financial Action Task Force.
Lee Sheppard of Tax Analysts, said: "Money laundering requires that the defendant move money in the course of committing or abetting some other crime, called a predicate offence, specifically listed in the statute.
"So that means Switzerland would have to accuse HSBC of money laundering in connection with some other crime listed as a predicate offence. The prosecutor's office suggested that the bank might be accused of aggravated money laundering, which, in a nutshell, is systemic money laundering (Penal Code Art. 305bis)."
Aggravated money laundering
Aggravated cases of money laundering are those committed within a criminal organisation, or committed by a gang with the purpose of systematically carrying out money laundering, or if an important turnover is realised, or if money laundering is carried out as a trade or business, according to the Swiss Penal Code.
Serious cases of money laundering in Switzerland can mean a prison sentence of up to five years and fines of up to CHF1m (£684,400).
The prosecutor's office said HSBC could be accused of having failed to take measures to prevent commission of crimes within it. The company could be prosecuted regardless of the criminal liability of its employees if it failed to take all the organisational measures necessary to prevent infringements from occurring within it, said Sheppard.
It has been suggested that the criminal prosecution could be indicative of supervisory lapses on the part of FINMA, the Swiss financial regulator, said the Tax Analysts report.
Bradley Birkenfeld, the whistle-blower who first shone a light on large scale tax evasion at UBS, told Tax Analysts: "The Swiss government is complicit in the global tax scandal. This is not a serious investigation. They condoned and promoted it all these years, and now they're doing an investigation?"
Birkenfeld received a subpoena to appear in court in Paris on 27 February, 2015, to provide evidence on an ongoing French investigation into the activities of UBS.