Police in Geneva were looking for several suspects "on the basis of a terrorist threat," Swiss security officials said Thursday.
"In light of the international and national investigation that followed the attacks in Paris, the Police Department in Geneva is actively searching for several individuals under the guidance of the Federal Intelligence Services," according to a statement issued by the city's department of security.
Authorities got word Wednesday that federal officials had flagged that "several suspects were likely in Geneva or in the region around the city," it added.
A senior U.S. counter terrorism official told NBC News that the CIA tipped off the Swiss authorities.
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Police were boosting the threat level and reinforcing "agents on the ground," the statement said. Authorities did not specify how many people were being sought or their identities, but a later statement said the suspects were believed to be in violation of a ban on groups such as al Qaeda or ISIS.
U.N. security guards in Geneva, the European headquarters of the world body, were stationed with MP5 sub-machine guns at entry points for cars, a highly unusual measure at the sprawling complex near the French border.
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A security guard at the U.N. site told Reuters that Swiss authorities were searching for four people believed to be in the city, or nearby. Another guard said the U.N. compound was on maximum alert. NBC News was not immediately able to independently verify that report.
The investigation is focused on a vehicle with Belgian license plates that was seen twice between Tuesday evening and Wednesday morning, Swiss news website 20 Minutes reported citing sources.
Shooting and bomb attacks claimed by ISIS killed 130 people in Paris on Nov. 13. Police in several European countries are still searching for individuals believed to have been involved in the attacks or in their planning.
Officials said Wednesday they didn't believe this latest threat was directly related to the Paris attacks.