A child holds a pen during a vigil outside the Consulate General of France to pay tribute to the victims of an attack on satirical magazine Charlie Hebdo in Paris
IN PHOTO: A child holds a pen during a vigil outside the Consulate General of France to pay tribute to the victims of an attack on satirical magazine Charlie Hebdo in Paris, in San Francisco, California January 7, 2015. The youngest of three French nationals being sought by police for a suspected Islamist militant attack that killed 12 people at a satirical magazine on Wednesday turned himself in to the police, an official at the Paris prosecutor's office said. The hooded attackers stormed the Paris offices of Charlie Hebdo, a weekly known for lampooning Islam and other religions, in the most deadly militant attack on French soil in decades. Reuters/Stephen Lam

The United States is waging war against "lone wolf" terrorists following the deadly terror attacks in France that led to the deaths of 17 people. About a million people had gathered in Paris to remember the victims of terrorism and express defiance amid extremists' calls for Western attacks.

U.S. Attorney General Eric Holder, one of the people who attended a counter-terrorism summit and rally in Paris, told NBC that he is often thinking about the undetected "lone wolf." He echoes the worldwide sentiment that extremists "corrupt the Islamic faith" to justify their acts of violence.

When asked why such people escape the authorities' detection and manage to attack, Holder said those individuals are often "under the radar." The White House has recently announced that the U.S. will be hosting an anti-extremism conference on Feb. 18. Holder remarked that the summit will bring together allies to discuss ways to prevent violent extremism.

Holder reiterated that the U.S. was "horrified" at the terror attacks and its citizens express their condolences to the people of France. He said the world has seen terrorism in the U.S., UK, Nairobi, Canada and Australia. Holder believes lone wolf attacks have become new threats that the world must confront through sharing of information.

Following the Paris attacks, a firebomb was recently thrown at the offices of a German newspaper after it republished the satirical cartoons of Charlie Hebdo. According to the Guardian, the responsibility for the attacks remains unclear but a video released two days after the death of Amedy Coulibaly, who shot dead four hostages in the grocery and a policewoman, revealed that he swore allegiance to ISIS. Coulibaly had coordinated with the Charlie Hebdo attackers and had been shot dead by French police.

The undated video was posted on Twitter by an anonymous supporter of extremism showing Coulibaly addressing the camera with a black flag associated with radicalism in the background. The video also showed several guns while he pledged his allegiance to the "emir of the faithful Abu Bakr Al-Baghdadi" and the leader of ISIS.

Meanwhile, ISIS has renewed calls for more attacks in the U.S. and other Western countries days after the terror attacks in Paris. The terrorist organisation released a video calling for others like the Charlie Hebdo attackers to target intelligence and law enforcement authorities and civilians.

After the praising the siege in a Lindt café in Sydney, Australia, ISIS has expressed its approval for the "lions of Islam" who killed 17 people in Paris. ABC reported that federal and local authorities have been asked to remain alert in the coming days. The FBI and the Department of Homeland Security are expected to release a joint intelligence bulletin for law enforcement agencies in the U.S. to stay vigilant. The latest ISIS video had cited the U.S., France, Canada and Australia as the specific targets of terror attacks.