The investigation was opened “several weeks” before the October 7 attacks by Hamas on Israel, Stefan Blaettler said, on Swiss public radio station SRF, without revealing further details.

The Attorney General’s office later told AFP that the investigation was launched “on suspicion of financing Hamas from Switzerland”.

The investigation is expected to be laborious because, unlike the European Union and United States, Switzerland has not placed bans on Hamas. But since the attack, mounting calls for the Swiss government to take action has prompted it to respond.

Four days after the attack, the government said it was “of the opinion that Hamas must be classified a terrorist organisation”.

A Middle East task force, established in the wake of the attacks, has been tasked with studying options for classifying the Islamist militant group as “terrorist”, although the government has given no timeline on the matter.

Swiss President Ignazio Cassis had two days after the attack argued that the government can only ban organisations that are also banned by the United Nations.

As a result, only Al-Qaeda and the Islamic State group as well as a few related organisations are currently banned in Switzerland.

All the attempts to ban Hamas in parliament have so far failed but a committee in the lower house recently put forward a new proposal.

Hamas militants stormed into Israel from the Gaza Strip two weeks ago and killed at least 1,400 people, mostly civilians who were shot, mutilated or burnt to death on the first day of the raid, according to Israeli officials.

Israel says around 1,500 Hamas fighters were killed in clashes before its army regained control of the area under attack.

More than 4,300 Palestinians, mainly civilians, have been killed across the Gaza Strip in relentless Israeli bombardments in retaliation for the attacks by the Palestinian Islamist militant group, according to the latest toll from the Hamas health ministry in Gaza.