Two men of Syrian origin have been arrested in Geneva on suspicion of making explosives and having links to extremist groups, Switzerland's attorney general's office announced Saturday.

Prosecutors said that a criminal proceeding has been opened against the pair for the manufacturing, concealing and transporting of explosives, as well as on suspicion of violating a federal law prohibiting "groups like al Qaeda, [ISIS] and similar organizations."

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The men were originally thought to have also been transporting toxic gas, but prosecutor Olivier Jornot said during a news conference Saturday that none was found in their car.

Jornot said the two suspects had arrived in Geneva shortly before they were detained Friday.

A senior U.S. counter terrorism official told NBC News that the CIA tipped off the Swiss authorities.

The news of the arrest came two days after police in the city said they were looking for several suspects "on the basis of a terrorist threat."

"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," said a statement by the city's department of security issued Thursday.

Authorities got word Wednesday that federal officials had flagged that "several suspects were likely in Geneva or in the region around the city.”

Jornot said more arrests could be expected in the coming days, but no link has been established between the larger investigation and the arrest of the two men from Syria on Friday.