GENEVA -- Working on a tip from the CIA, Geneva police were hunting for at least four suspects allegedly linked to the radical Islamic State group and believed to be plotting a "specific" attack in the city, Swiss officials said Thursday.

As city police raised their security alert level, Swiss state prosecutors opened investigations into a suspected criminal plot and suspected violations of a ban on groups such as Islamic State and al-Qaida.

The CIA alerted Swiss authorities to the four men Wednesday, prompting the manhunt, a Swiss official with knowledge of the investigation said Thursday. He spoke on condition of anonymity because of the sensitivity of the matter.

The official confirmed the suspects were four men shown in a photograph linked to a news report in Swiss daily Le Matin. Each was pictured in a crouched pose and holding up an index finger -- said to be an Islamic State gesture.

It was too early to determine whether other suspects still might be at large, the official said. He also declined to give the names, ages or nationalities of the people being sought.

The Geneva security department said Swiss national authorities flagged "suspicious individuals" who could be in Geneva or the vicinity to city authorities Wednesday. It didn't specify how many people were being sought or name them.

The official said, however, the suspects were not believed to have had links to the Paris terror attacks, appearing to revise an earlier statement from the Geneva security department.

Heavily armed police carrying submachine guns stood guard outside diplomatic missions in Geneva on Thursday. Normally, officers carry lighter weapons. Geneva police also were deploying more officers at the city's train station and airport.

The U.N. office in Geneva was also increasing its security precautions, even though it said there was no indication of any specific increased threat against U.N. staff.