Nice, France. REUTERS/Eric Gaillard It's not clear yet who is responsible for the truck attack that killed dozens at a Bastille Day celebration in Nice, France. But terrorist groups have long been calling for supporters to attack "infidels" with cars.

At least 80 people were killed in the southern French city of Nice when a truck ran into a crowd celebrating the country's national holiday on Thursday night.

The earliest information from the attack does point to terrorist involvement. US President Barack Obama said that it appears to be a "horrific terrorist attack."

The truck was reportedly loaded with firearms and grenades, and US officials told The Daily Beast that the terrorist group ISIS — aka the Islamic State, ISIL, or Daesh — is a top suspect in the attacks.

ISIS and Al Qaeda have publicly called for supporters to use vehicles as weapons.

The Institute for the Study of War noted in a 2014 report that ISIS spokesman Abu Mohammad al-Adnani instructed supporters in a speech in September of that year.

He said:

"If you are not able to find an IED or a bullet, then single out the disbelieving American, Frenchman, or any of their allies. Smash his head with a rock, or slaughter him with a knife, or run him over with your car, or throw him down from a high place, or choke him, or poison him."

And a 2014 ISIS video aimed at French-speaking recruits encouraged supporters to attack people in France with cars and other easily accessible weapons.

"If you are unable to come to Syria or Iraq, then pledge allegiance in your place — pledge allegiance in France," a French ISIS member says in the video. "Operate within France."

The man then goes on to mention cars specifically: "There are weapons and cars available and targets ready to be hit. ... Kill them and spit in their faces and run over them with your cars."

Al Qaeda has also put out global calls to attack Westerners with cars.

In the second issue of its English-language magazine Inspire, the terrorist group referred to pickup trucks as "the ultimate mowing machine."

"The idea is to use a pickup truck as a mowing machine, not to mow grass but mow down the enemies of Allah," the magazine article states.

Pro-ISIS accounts on the messaging app Telegram, which the terrorist group uses as a platform to disseminate its message, have been celebrating the Nice attack. But the group has yet to make any claim of responsibility.

ISIS, in particular, has increasingly been relying on external attacks as it has been losing territory in the Middle East, where its self-declared "caliphate" lies.

When the terrorist group first rampaged across Iraq and Syria claiming territory, it encouraged supporters to travel to the Islamic State, but recently ISIS rhetoric has shifted to focus on encouraging people to mount attacks in their home countries.

Sometimes these attacks are directed by ISIS leadership, but sometimes they are carried out by lone actors who don't have any significant contact with ISIS members.

Mia Bloom, a terrorism expert at Georgia State University, told Business Insider that it's too soon to tell who's responsible for the Nice attack.

She wrote in an email:

"It is true that Isis has returned many fighters to France for these kinds of attacks. It is equally true that if Al Qaeda felt ignored it might plan an elaborate operation to get itself back in the media spotlight and back on the map. My research showed groups might compete with each other for ever-[more] spectacular attacks."