At the end of March, authorities "arrested three Iraqi citizens suspected of providing support to the criminal organization known as the Islamic State (Isis)," Switzerland's attorney general said in a statement.
"They are thought to have been planning a terrorist attack in Europe," the statement said.
Swiss news media reported the arrests at the end of September, saying Switzerland's Federal Intelligence Service had been tipped off by a Western counterpart which intercepted the men's phone calls.
The trio, according to media reports, allegedly helped around 40 jihadists residing in Switzerland to travel to the Middle East to join Isis.
They also purportedly sought to procure explosives and toxic gas for an attack.
The attorney general's office said Friday the men were suspected of providing support to the Islamic State organization and of planning attacks with explosives and toxic gases.
It also said the three were suspected of encouraging illegal entry into Switzerland and residency in the wealthy Alpine nation.
Swiss investigators, with assistance from European and US authorities, were attempting to identify possible accomplices and uncover networks both in Switzerland and elsewhere, the statement said.
It added that Switzerland's top prosecutor was currently overseeing 20 probes linked to jihadist activity.