In the wake of the Paris attacks, politicians in the United States and Europe are claiming that the movement of refugees and migrants across Europe's borders made it possible for Islamic State (ISIS) militants to infiltrate the continent.

In response, Europe is tightening its borders, and the refugee crisis has become an unlikely subject in next year's U.S. election. So far, Donald Trump has questioned whether Syrian refugees seeking asylum in America were part of a "Trojan Horse" plot and Gov. Chris Christie has suggested that even 5-year-old Syrian orphans should be kept out of the U.S. There have also been calls to discriminate among refugees according to religion.

But while investigators are still unravelling the plot, one fact is clear at this point: Not one Paris attacker has been officially identified as a refugee.

See also: What you need to know about Syrian refugees

The eight people officially named in connection with the Paris attacks were all European citizens who held European passports, allowing them to move in and out of Europe with a high degree of freedom.

At least 130 people were killed in the Nov. 13 attacks and hundreds more were injured.

"And which do you prefer? Syrian immigrants in France? Or French immigrants in Syria?" A very good question. pic.twitter.com/KRj6zDIpYM — Liz Sly (@LizSly) November 21, 2015

The identity of the attackers

Speculation that led some to link the Paris attacks to the refugee crisis was spurred by the discovery of a Syrian passport near one of the suicide bombers. The passport was issued in the name of Ahmad Al-Mohammad, a 25-year-old from the rebel-held Syrian city of Idlib.

French officials have said that the bomber's fingerprints match a set taken in Greece in October when a person holding Al-Mohammad's passport entered the Greek island of Leros from Turkey. The man later entered Serbia, where authorities took fingerprints matching those taken in Greece. The following day, the man crossed into Croatia, according to a Serbian security official who spoke to The Associated Press.

However, it is not known whether the suicide bomber was indeed Al-Mohammad or someone traveling with a fake Syrian passport, which have become legion inside and outside Europe.

A third stadium bomber hasn't been named, but French officials said he entered Greece at the same time as Al-Mohammed.

All of the attackers and accomplices named thus far are all European citizens. Here is what we know about each.

Abdelhamid Abaaoud, 27

Abdelhamid Abaaoud, the 27-year-old described as the "ringleader" behind the attacks, was Belgian-born and of Moroccan descent. He left his home country by choice and traveled into the Syrian conflict zone to take up arms with Islamic State militants.

He was killed by French police during a raid on Wednesday in Paris' Saint-Denis neighborhood, and his death has left some unanswered questions. It is still unclear how and when Abaaoud entered France before his death. In a January interview in Islamic State group's English-language magazine Dabiq, he bragged that he was able to slip in and out of Europe undetected.

Hasna Aitboulahcen, 26

A female accomplice, Hasna Aitboulahcen, was Abaaoud's cousin. She was born in the Paris suburb of Clichy-la-Garenne, and doesn't appear to have spent time Syria or Iraq.

Salah Abdeslam, 26

26-year-old Salah Abdeslam is believed to have taken part in the Paris attack, and is still on the run from police. He was born in Brussels.

Ibrahim Abdeslam, 30

His Belgian-born brother, 30-year-old Ibrahim Abdeslam, detonated a suicide vest at cafe Comptoir Voltaire as part of the attacks.

Ismael Omar Mostefai, 29

Two of three men who laid siege to the Bataclan concert hall, and died during the attack, were identified as European citizens. Ismael Omar Mostefai, 29, was a Frenchman of Algerian origin, who police say detonated a suicide vest at the theater.

Samy Amimour, 28

Frenchman Samy Amimour, 28, also opened fire on concertgoers in the Bataclan. He has been previously charged in a terrorism investigation in 2012, but dropped off the radar and was the subject of an international arrest warrant. A third theater attacker remains unidentified by police.

Ahmad Dahmani, 26

Another Belgian man of Moroccan origin, Ahmad Dahmani, was arrested in Turkey on Saturday on suspected involvement with the Paris attacks.

Bilal Hadfi, 20

Bilal Hadfi, a French citizen who lived for some time in Brussels, has also been identified as one of the three bombers at the Stade de France.

The reality for refugees

As winter bears down on Europe and temperatures drop, the refugee situation is becoming dire. Tens of thousands are still making their way across the continent. UNHCR says lack of resources at border crossings leave those refugees at risk.

UNHCR has warned that halting the flow of refugees across EU borders will worsen the humanitarian crisis.

"This is becoming increasingly untenable from every point of view – humanitarian, legal, and also safety related, not least in light of falling temperatures and the risks for children and others with specific needs," read a statement from UNHCR issued on Friday.

In the U.S., lawmakers are also pushing legislation to tighten screening requirements for Syrian refugees; some Republican presidential candidates want to halt their entry to the country entirely and some governors have penned open letters to the president saying their state will not resettle any Syrians.

Refugees walk over a snowy field at the Austrian-German border near Hanging, Austria, November 22, 2015. Image: Armin Weigel/Associated Press

But President Barack Obama has stayed true to his promise of accepting 10,000 Syrian refugees into the country next year. While he said that Americans are "right to be concerned" in the wake of the attacks, he asserted that the country must not surrender to fears "that lead us to abandon our values, to abandon how we live."

The resettlement process for Syrian refugees in the U.S. is already quite stringent and includes the collection of biometric data, extensive interviews and background checks.

Additional information from the Associated Press.

