(CNN) Geneva, Switzerland, was on high alert Thursday as police searched for suspects related to the terror attacks in Paris , a source close to the investigation told CNN.

French security forces move people in the area of Rue Bichat in the 10th District. A witness told BFMTV that firefighters were on the scene to treat the injured.

French security forces move people in the area of Rue Bichat in the 10th District. A witness told BFMTV that firefighters were on the scene to treat the injured.

Rescue workers and medics tend to victims at the scene of one of the shootings, a restaurant in the 10th District. Attackers reportedly used AK-47 automatic weapons in separate attacks across Paris, and there were explosions at the Stade de France.

Rescue workers and medics tend to victims at the scene of one of the shootings, a restaurant in the 10th District. Attackers reportedly used AK-47 automatic weapons in separate attacks across Paris, and there were explosions at the Stade de France.

People leave the Stade de France after explosions were heard near the stadium during a soccer match between France and Germany on Friday. Paris Deputy Mayor Patrick Klugman told CNN President Francois Hollande was at the match and was evacuated at halftime.

People leave the Stade de France after explosions were heard near the stadium during a soccer match between France and Germany on Friday. Paris Deputy Mayor Patrick Klugman told CNN President Francois Hollande was at the match and was evacuated at halftime.

French security forces rush in as people are evacuated in the area of Rue Bichat in the 10th District of Paris.

French security forces rush in as people are evacuated in the area of Rue Bichat in the 10th District of Paris.

Spectators gather on the field of the Stade de France after the attacks. Explosions were heard during the soccer match between France and Germany.

Spectators gather on the field of the Stade de France after the attacks. Explosions were heard during the soccer match between France and Germany.

A wounded man is evacuated from the Stade de France in Saint-Denis, outside Paris.

A wounded man is evacuated from the Stade de France in Saint-Denis, outside Paris.

Police secure the Stade de France in Saint-Denis, north of Paris, following explosions during the soccer match between France and Germany.

Police secure the Stade de France in Saint-Denis, north of Paris, following explosions during the soccer match between France and Germany.

A riot police officer stands by an ambulance near the Bataclan concert hall in central Paris.

A riot police officer stands by an ambulance near the Bataclan concert hall in central Paris.

A medic tends to a wounded man following the attacks near the Boulevard des Filles du Calvaire.

A medic tends to a wounded man following the attacks near the Boulevard des Filles du Calvaire.

Wounded people are evacuated outside the scene of a hostage situation at the Bataclan theater in Paris on November 13.

Wounded people are evacuated outside the scene of a hostage situation at the Bataclan theater in Paris on November 13.

Rescuers evacuate an injured person on Boulevard des Filles du Calvaire, close to the Bataclan concert hall in central Paris.

Rescuers evacuate an injured person on Boulevard des Filles du Calvaire, close to the Bataclan concert hall in central Paris.

A body, covered by a sheet, is seen on the sidewalk outside the Bataclan theater.

A body, covered by a sheet, is seen on the sidewalk outside the Bataclan theater.

Spectators embrace each other as they stand on the playing field of the Stade de France stadium at the end of a soccer match between France and Germany in Saint-Denis, outside Paris, on November 13.

Spectators embrace each other as they stand on the playing field of the Stade de France stadium at the end of a soccer match between France and Germany in Saint-Denis, outside Paris, on November 13.

Spectators invade the pitch of the Stade de France stadium after the international friendly soccer match between France and Germany in Saint-Denis.

Spectators invade the pitch of the Stade de France stadium after the international friendly soccer match between France and Germany in Saint-Denis.

A survivor of the terrorist attack in the Bataclan is assisted following terror attacks, November 13. The violence at the Bataclan, which involved a hostage-taking, resulted in the highest number of casualties of all the attacks.

A survivor of the terrorist attack in the Bataclan is assisted following terror attacks, November 13. The violence at the Bataclan, which involved a hostage-taking, resulted in the highest number of casualties of all the attacks.

Rescuers evacuate an injured person near the Stade de France, one of several sites of attacks November 13 in Paris. Thousands of fans were watching a soccer match between France and Germany when the attacks occurred.

Rescuers evacuate an injured person near the Stade de France, one of several sites of attacks November 13 in Paris. Thousands of fans were watching a soccer match between France and Germany when the attacks occurred.

Forensics are working in the street of Paris after the terrorist attack on Friday, November 13. The words "horror," "massacre" and "war" peppered the front pages of the country's newspapers, conveying the shell-shocked mood.

Forensics are working in the street of Paris after the terrorist attack on Friday, November 13. The words "horror," "massacre" and "war" peppered the front pages of the country's newspapers, conveying the shell-shocked mood.

Victims of the shooting at the Bataclan concert venue in central Paris are evacuated to receive medical treatment on November 14.

Victims of the shooting at the Bataclan concert venue in central Paris are evacuated to receive medical treatment on November 14.

A man with blood on his shirt talks on the phone on November 14. He is next to the Bataclan theater, where gunmen shot concertgoers and held hostages until police raided the building.

A man with blood on his shirt talks on the phone on November 14. He is next to the Bataclan theater, where gunmen shot concertgoers and held hostages until police raided the building.

Police, firefighters and rescue workers secure the area near the Bataclan concert hall on November 14.

Police, firefighters and rescue workers secure the area near the Bataclan concert hall on November 14.

Medics evacuate an injured woman on Boulevard des Filles du Calvaire near the Bataclan early on November 14.

Medics evacuate an injured woman on Boulevard des Filles du Calvaire near the Bataclan early on November 14.

Shoes and a bloody shirt lie outside the Bataclan concert hall on November 14. Most of the fatalities occurred at the Bataclan in central Paris.

Shoes and a bloody shirt lie outside the Bataclan concert hall on November 14. Most of the fatalities occurred at the Bataclan in central Paris.

Police are out in force November 14 near La Belle Equipe, one of the sites of the terror attacks.

Police are out in force November 14 near La Belle Equipe, one of the sites of the terror attacks.

A forensic scientist works near a Paris cafe on Saturday, November 14, following a series of coordinated attacks in Paris the night before that killed scores of people. ISIS has claimed responsibility.

A forensic scientist works near a Paris cafe on Saturday, November 14, following a series of coordinated attacks in Paris the night before that killed scores of people. ISIS has claimed responsibility.

Police were looking for five radicalized people suspected to be part of Mourad Fares' network, the source said.

Fares, a French national, is the man who recruited Foued Mohamed-Aggad, who was identified as the third attacker at the Bataclan theater in Paris, the source said.

According to French Interior Minister Bernard Cazeneuve, Fares is a known recruiter for jihad in Syria . French intelligence sources say he's worked for ISIS , mainly in Francophone countries, and in Switzerland, including in Geneva.

He was arrested in August 2014 in Turkey and was placed in detention in France on September 11, 2014.

U.S. intelligence services provided information to Switzerland regarding four individuals plotting attacks against Switzerland and cities in the United States, the source said.

Swiss authorities then raised the terror alert level as a precaution, according to the source. Authorities in Geneva don't know whether these individuals are currently on Swiss territory, the source said.

National authorities gave Geneva authorities a description of suspects who could be in Geneva or the Geneva region. Investigations are being carried out in close collaboration with international and French police.

The source specified three factors that triggered the alert.

One was the information provided to Swiss authorities from U.S. intelligence agencies that recently intercepted communications of a group of extremists discussing the idea of attacking Geneva.

The extremists are believed to have returned to Europe after connecting with ISIS in Syria. The extremists also discussed the idea of launching attacks in Chicago and Toronto, the source said.

Second was information that an associate of Salah Abdeslam, a person suspected of having involvement in the Paris attacks, had crossed into Switzerland. Abdeslam is still at large.

And third was the identification of the third attacker at the Bataclan theater, Mohamed-Aggad. The link between a Paris attacker and the recruiter Fares generated security concerns about a Swiss associate of Fares suspected of traveling to Syria and joining with ISIS. His current whereabouts are unknown, the source said.

"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. She would not comment on the nature of the threat.

A statement said that the "level of vigilance" in the city has been raised and that more police have been put on the streets.

A spokesman at the United Nations complex in Geneva , Rheal LeBlanc, confirmed to CNN that U.N. security staff at entry points were armed with heavier equipment than normal.

Photos: Political violence is driving terrorism Photos: Political violence is driving terrorism A Nigerian policeman inspects the site of a suicide attack by Boko Haram at a busy cattle market in the northeastern Nigerian city of Maiduguri in June, 2015. Boko Haram overtook ISIS as the world's deadliest terror group last year, according to the Global Terrorism Index, while Nigeria had the biggest year-on-year increase in terrorism, with deaths up more than 300%. Hide Caption 1 of 6 Photos: Political violence is driving terrorism In this image taken from social media, an ISIS fighter holds the group's flag as he stands on a tank, purportedly captured when they took over the town of Qaryatain, Syria. ISIS, along with Boko Haram, is one of two groups accounting for 51% of claimed terrorism deaths in 2014. Hide Caption 2 of 6 Photos: Political violence is driving terrorism A member of the Iraqi security forces fires ammunition in Baghdad in 2014 during a funeral procession of an Iraqi politician. The deadliest city in the world for terrorism is Baghdad. There were 2,454 deaths in Baghdad in 2014, with a death rate from terrorism of 43 per 100,000 people. Hide Caption 3 of 6 Photos: Political violence is driving terrorism Iraqi counter-terrorism forces patrol a street in Tikrit in April 2015, a day after the country's prime minister declared victory in the battle to retake the city from ISIS. Hide Caption 4 of 6 Photos: Political violence is driving terrorism An Iraqi Kurdish Peshmerga fighter prepares an ammunition belt as he guards a position at the frontline of fighting against ISIS militants near the northern Iraqi town of Sinjar. Hide Caption 5 of 6 Photos: Political violence is driving terrorism A Syrian family waits after arriving on the Greek island of Lesbos along with other migrants and refugees, on November 17, 2015. Ten of the 11 countries most affected by terrorism also have the highest rates of refugees. Hide Caption 6 of 6

Security measures were increased because of advice from Swiss authorities, LeBlanc said. He said no extra security staff has been added.

The Geneva complex is the second-largest U.N. center in the world, after the headquarters in New York City.

Eight attackers were killed in the November 13 Paris attacks, and two suspects are believed to be at large. They are being sought across Europe.

Abdeslam, 26, was driving toward the Belgian border hours after the attacks when police stopped and questioned him. Authorities have not said what role he may have played in the attacks.

Mohamed Abrini, 30, now the subject of an international arrest warrant , drove a car that was found abandoned in a Paris neighborhood where one of the attacks occurred. Authorities said he had dropped off one of the bombers who attacked the Stade de France.

Last month, terrorists armed with assault rifles and explosives targeted several locations across Paris, including busy restaurants, a concert hall and the Stade de France, where a soccer game between France and Germany was being played.

They killed 130 people and wounded hundreds. ISIS has claimed responsibility.