It began as a vague threat.

Two Syrian men in greater Geneva, Switzerland, were arrested on Saturday following the suspicion of the manufacture, concealment and transport of explosives or toxic gases.

But the threat quickly went from “vague” to “direct” when authorities found traces of explosives in the men’s car. Authorities have arrested the men and are hunting for additional suspects who have indirect links to suspects from the terrorist attacks in Paris. It remains unclear whether the men in Switzerland were linked with Paris, however.

The men arrested came from Syria, and did not speak French, the Guardian reported. Earlier this week, Swiss police announced they were searching for four men with alleged ties to ISIS.

The alert reportedly came from US intelligence officials who informed their Swiss counterparts they’d intercepted a conversation about a plot to attack Switzerland. More arrests are expected in the coming days.

The US Embassy in Bern warned U.S. citizens “to review their personal security plans; remain aware of surroundings at all times, and monitor local news stations for updates. We also recommend maintaining a high level of vigilance and take appropriate steps to enhance personal security.”

Switzerland remains on high alert, and as local Swiss media reported, “the threat level is actually up a notch.” Policemen are arming the streets with machine guns in front of United Nations institutions, airports, train stations. UN buildings were evacuated on Wednesday night following the threat that a suspect van with Belgian license plates was seen in Geneva on Dec. 8.

“We have gone from a vague threat to a precise threat,” Emmanuelle Lo Verso, head of communications at the Geneva Department of Security told CNN.

Authorities have announced the threat in Switzerland has been more specific than the threats to other cities, such as Chicago or Toronto.

“The intelligence about Geneva was stronger, more specific than it was about the other cities because you’re simply not hearing about that kind of reaction from either Toronto or Chicago at this stage,” Juliette Kayyem, national security analyst, told CNN.

The threats in Switzerland are what some are calling “unprecedented.”

“To my knowledge, and after twenty years of experience in intelligence and counterterrorism, this is the first time I have seen such a serious threat in Switzerland,” said Jean-Paul Rouiller, creator of the counterterrorism unit of the Swiss Federal Judicial Police, according to Breitbart.