(CNN) Not one, but two French citizens have been connected to beheadings in a recent ISIS video, both of them 22-year-old men who went to Syria to join the group, authorities said Wednesday.

The Paris prosecutor's office on Wednesday identified Mickael Dos Santos, a convert to Islam from the Paris suburb of Champigny-sur-Marne who was well-known to French anti-terror authorities before the ISIS video release.

In fact, an October 2013 warrant for Dos Santos' arrest was issued as part of an investigation into French citizens who had gone to Syria to take part in ISIS jihad there, the prosecutor's office said.

Dos Santos joins Maxime Hauchard, who authorities said Monday was also on the video, released over the weekend, that depicts in graphic detail, the beheadings of men whom ISIS militants claim were Syrian government pilots. It also shows the aftermath of another beheading in which the victim in not clearly recognizable, but that the U.S. government says was American aid worker Peter Kassig

Public Prosecutor Francois Molins described Hauchard -- who went to Syria in 2013 and visiting Mauritania the previous year -- is a "self-radicalized" jihadist who traveled to the region under the guise of a humanitarian mission. He was known to French security services as far back as 2011, the prosecutor said.

Speaking from Australia alongside Prime Minister Tony Abbott, French President François Hollande said his country's judicial system will work to establish "exactly what role" the two French men played, though the mere fact they came from France and became involved at all warrants attention and action.

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"We have to be even more aware, even in terms of the information that needs to be handed out, in terms of the danger that Internet sites carry and how these young people can be indoctrinated," Hollande said. "They could be from any background, from any ethnic origin, but they easily can be brainwashed into becoming converts."

ISIS foreign fighters prompt concern

The ranks of ISIS -- the extremist group calling itself the Islamic State that's taken over vast swaths of Syria and Iraq -- have swelled with volunteers coming from outside the region, including from the West.

Intelligence estimates indicate that more than 100 of the foreign fighters in Iraq and Syria have come from the United States. Hundreds more arrived from Europe, including more than 900 French citizens that country's government believes are involved in the jihad there. This influx has spurred concerns that some of these fighters could bring the fight back home, perhaps with attacks inside Western nations.

Those worries have been heightened by the group's brutal tactics against foe and civilians alike, such as raping, enslaving and selling Iraqi women belonging to the Yazidi religious minority. Last week, a United Nations panel stated what many consider obvious: ISIS has committed war crimes and crimes against humanity in Syria.

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The most high-profile of these abuses -- because they were taped and publicized as propaganda, for all the world to see -- have been the beheadings of ISIS captives. The mother of the first American so killed, journalist James Foley, said that her family is in touch with Kassig's family and that they are "heartbroken another American has given his life for the suffering people of Syria."

The West is fighting back, with many countries -- including France -- allying with Iraq to conduct airstrikes targeting ISIS in that country. The United States has also led airstrikes going after the group in Syria.

From Monday to Wednesday, for instance, the U.S.-led coalition launched five airstrikes near the northern Syrian city of Kobani and one targeting an oil collection point in al-Hasakah, in addition to a strike targeting a facility purportedly used by the so-called Khorasan Group. There were even more strikes in Iraq, including 13 near Kirkuk, seven near Mosul, three near Baiji and one near Fallujah, according to the U.S. military.

Official: ISIS carries out suicide bombing in Irbil

ISIS hasn't relented much either, including unleashing yet more violence Wednesday.

At least four people died and 29 others were wounded after a vehicle drove up to the entrance to a government building in Irbil, the strongly fortified capital of Iraqi Kurdistan, and exploded, according to Kurdistan Regional Government spokesman Safeen Dizayee. Police "who tried to stop" the vehicle were among the dead, he said.

Dizayee added that ISIS -- which is active in northern Iraq -- claimed responsibility for the attack.

Meanwhile, fighting continues in Syria as well, some of it near the Turkish border.

John Allen, the former commander of American forces in Afghanistan now charged with coordinating the coalition to battle ISIS, met with officials in the Turkish capital of Ankara about that fight on Wednesday.

The same day, Turkish President Recep Tayyip Erdogan called for more to be done -- including establishing no-fly zones and bolstering "train and equip" efforts for those fighting ISIS. Turkey has spoken out against ISIS, while also expressing opposition to groups of Kurdish fighters that are among those trying to defeat the group.

"The coalition powers still have not taken these steps we asked for and recommended," Erdogan said, according to a report from the semi-official Anadolu news agency. "We are living this process on small hints and possibilities."