There is a possibility that suspects directly involved in Friday's Paris terror attacks remain at large, a French counterterrorism source close to the investigation told CNN on Sunday.

NBC Montana reports a number of arrests linked to the attacks have made in Belgium, but it is unclear whether they include the occupant or occupants of an abandoned car with weapons found in eastern Paris, the source said.

[Original story published at 10:03 a.m. ET]

Europe's worst security nightmare appears to be coming true: At least one of the terrorists who attacked civilians in Paris on Friday entered the European Union hidden among the wave of refugees arriving on European shores.

One of three bombers who detonated himself at the Stade de France late Friday arrived on the Greek island of Leros on October 3 among numerous Syrian refugees, CNN's Christiane Amanpour reported, citing an unnamed French senator who was briefed by the Ministry of the Interior.

The man declared himself to be Syrian, said his name was Ahmad al Mohammad and was, under new procedures set up to help refugees, issued a new emergency passport or similar document.

From Leros, he traveled to Macedonia, Serbia and then Croatia, Amanpour reported.

The fingerprints from the bomber at the Stade de France match those taken when the man was issued his emergency travel document on Leros.

The two others who detonated themselves at the stadium carried false Turkish passports, Amanpour reported.

It appears that the Islamic extremist group ISIS is making good on its pledge to hide operatives among the tide of legitimate refugees now entering Europe.

European officials believe, Amanpour said, that there is "a very professional new squad of terrorists inserting themselves into some of these migrant voyages."

ISIS has claimed responsibility for Friday night's coordinated series of deadly assaults, the worst violence in France since World War II.

Three Kalashnikovs found in abandoned car

Investigators in France and elsewhere have begun making arrests as they begin to uncover the identities and backstories of the terrorists who killed more than 120 people in gun and bomb attacks across Paris.

Clues and evidence appeared to be piling up Sunday. The French channel BFMTV, a CNN affiliate, reported that authorities had found three Kalashnikov automatic rifles in an abandoned car in the eastern Paris suburb of Montreuil that was believed to have been used by perpetrators of the attacks.

At least seven people have been arrested in raids in Belgium connected to the Paris attacks, officials said. Those arrested were in contact with the Paris attackers, a senior Belgian counterterrorism source told CNN. No weapons or explosives were found on them.

Two attackers have been identified as French citizens who lived in the Molenbeek district of Brussels, according to Jean Thoreau, spokesman for Belgian Federal prosecutor, who spoke to CNN on the phone.

But officials are still trying to figure out who the assailants were and how they planned and carried out near-simultaneous massacres at restaurants, bars and a concert hall in the French capital without being detected beforehand by intelligence agencies.

One of the suicide bombers has been identified as Ismael Omar Mostefai, according to Jean-Pierre Gorges, the mayor of the French town of Chartres, who is also a member of Parliament. Mostefai lived in Chartres at least until 2012, Gorges said in a Facebook post Saturday.

One attacker identified as French

The news agency Agence France-Presse reported that Mostefai was the same attacker who was described Saturday by Paris prosecutor Francois Molins, something CNN has not confirmed independently.

Molins, who did not identify the attacker by name, said he was a 29-year-old French citizen with a criminal record from the southern Paris suburbs.

The attacker was involved in the assault and hostage-taking at the Bataclan concert hall, where the highest number of people were killed Friday, according to the prosecutor.

The man, who was identified by fingerprints, was believed to have been radicalized in 2010 but had never been accused of terrorism, Molins said.