Based on what is known so far, Muhammad Youssef Abdulazeez was born in Kuwait and immigrated to the United States with his family as a young boy. He became an all-American boy in Chattanooga, Tenn. He grew up in a a somewhat strict Sunni Islamic family, but there don't seem to have been any signs he was an extremist — at least not until he had already killed four U.S. Marines in a rampage gun attack on a recruitment station and a Navy Reserve center on Thursday.

We know now that Abdulazeez was an engineer, having graduated from a state college. He had wrestled in high school and loved mixed martial arts fighting. He had personal and family problems, and he suffered his first run-in with the law earlier this year, when he was cited for driving while intoxicated. He had recently traveled to the Middle East and visited Yemen.

Investigators are right not to jump to conclusions, but they are also right to treat this as a terrorist act until proven otherwise.

It certainly has all the trappings of terrorism. It would be impossible to argue that Abdulazeez chose his targets at random — he attacked two separate military installations that were seven miles apart from each other. Such an attack is clearly meant to send a message about the U.S. military.

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This would not be inconsistent with previous instances of Islamic terrorism against uniformed military personnel within the United States. Nor would this attack be unique in its specifics. In 2009, a Muslim convert who had taken the name Abdulhakim Mujahid Muhammad attacked a military recruitment center in Little Rock, Ark., killing one and wounding another.

Like Abdulazeez, Muhammad had recently spent time in Yemen, a tumultuous country where terrorist networks are especially strong and American Muslims are prized as terrorist recruits. Muhammad's claims to have been directed by al Qaeda have never been verified (he had multiple psychological problems). But he was at least influenced by its ideology, which demands a very pure form of Islam, for whose sake violence is justifiable, and which views Western mores in general and the United States government in particular as major obstacles to that.

This attack has changed everything for those affected — in particular the families of the four Marines who were killed. But in a sense, it changes nothing. There are no new lessons beyond what Americans learned in September 2001. As then, a specifically radical and violent brand of Islam continues to threaten Americans within their own country, just as it threatens people around the world. In fact, as the Islamic State's rampage through Iraq and Syria shows, this ideology threatens Muslims most of all — especially the ones who belong to the wrong sect or find themselves a few ticks shy of whatever the most violent loon in the region decides is perfect observance of Islamic law.

Victory over such ideas — whose followers comprise a minority in the Islamic world, but a disturbingly large one — will be a work of generations, not years. In the meantime, the best that most Americans can do in response to an incident like this one to pray for the families of the families of the victims, and to find an appropriate and meaningful way to support the troops — perhaps by donating to a charity that serves service members or their families.

As with their brethren who died in combat on foreign battlefields, these Marines made the ultimate sacrifice because they dared to wear their country's uniform.