Terrorism is an age-old concept.

Some people think the first attack happened in the first century against Roman collaborators.

But modern terrorism is thought to come out of mid-19th century France, when good-quality, affordable explosives hit the market and radical political movements became more prominent.

It came to the U.S. during the same time frame.

Today, the terrorism landscape is much different, and so is the response.

The FBI says its No. 1 priority is protecting the U.S. from terrorist attacks, regardless of motivation.

Terrorism investigations fall into two categories at the FBI: international terrorism and domestic terrorism.

The first is when an attack is inspired by or associated with foreign organizations or nations.

If the attack or attacks are meant to further a domestic influence’s goals, it’s domestic terror.

The FBI’s examples of domestic terror motivations include politics, religion, race and social issues.

Domestic terror “remains persistent” overall, according to the FBI.

The FBI says people cross the line from exercising First Amendment freedoms to committing crimes in hopes of furthering violent agendas.

Terrorism has evolved immensely since 9/11.

The FBI says lone offenders and the internet have accelerated that evolution.

According to the FBI, lone offenders are tougher to identify and stop because they aren’t always clearly tied to a group.

And the internet — social media, in particular — helps terrorists connect with potential recruits more easily.

To meet the evolving threat, law enforcement is constantly changing its approach.