Friday morning, multiple news outlets reported some disturbing news: One of the San Bernardino shooters, 27-year-old Tashfeen Malik, apparently pledged allegiance to ISIS in a Facebook post before the shooting.

These reports cite unnamed federal law enforcement officials, so it's important to exercise caution about their veracity at this point. And even if they do turn out to be true, it's important to be cautious about what they mean: The fact that a shooter pledged allegiance to ISIS does not mean ISIS planned the attack. Nor does it necessarily mean that ISIS is the reason the attack happened.

What follows is a brief guide to what we know about the shooters' ISIS links — and what that tells us about the shootings themselves.

What we know about the reports

Before Friday, the evidence linking Malik and her husband/fellow shooter, Syed Rizwan Farook to Islamist extremism had been sketchy at best. As the Soufan Group, a private consulting firm that studies terrorism, explains, their stockpile of ammunition and bombs matches the pattern of terrorist groups — but the fact that Farook used to work in the same government department as the targets suggested a more personal motive.

But today's reports, from the New York Times and the Associated Press, are the best evidence yet that the shooting was in some way linked to ISIS.

Both pieces cited anonymous US law enforcement officials saying that Malik had posted a now-deleted pledge of allegiance to ISIS leader Abu Bakr al-Baghdadi on her personal Facebook page. The reports didn't disclose how federal authorities found it, though we know that they had been conducting an extensive investigation into the now-deceased shooters' electronic histories.

As the New York Times piece notes, "In the days leading up to the shooting, the couple in San Bernardino took several steps to delete their electronic information, in an apparent effort to cover their tracks, officials said. Those efforts have led authorities to believe that the shooting was premeditated."

ISIS asks people committing terrorist attacks in its name for such pledges — called "bayat" in jihadi parlance — before the attack. As they explain in the most recent issue of their English-language magazine, Dabiq:

Let him record his will, renew his bayat, carry the Khilfah banner, and strike the crusaders and their pagan and apostate allies wherever he can find them, even if he is alone.

"This MAY be what Tashfeen Malik did," Thomas Joscelyn, a senior fellow at the Foundation for the Defense of Democracies, tweeted. "Record her bayat (allegiance) for death."

Emphasis on "may" there: "[I] haven't seen her recording, so don't know for sure," Joscelyn concluded. More broadly, anonymous comments by federal officials to newspapers aren't definitive proof. So while Friday's reports are the strongest evidence yet of an ISIS link, we still cannot be certain that Malik and Farook were jihadis. The publicly available evidence is still too mixed.

What an ISIS pledge of allegiance means

If the message is real, it does not prove that the attack was directed by ISIS. According to the New York Times, US officials currently believe that the attackers had no links to ISIS headquarters in Iraq and Syria.

"At this point we believe they were more self-radicalized and inspired by the group than actually told to do the shooting," one of the officials told the Times.

This is an important distinction: There is a very big difference between random people choosing to take up the ISIS banner and ISIS having the organizational wherewithal to plan and execute attacks from its home base. Random individuals inspired by ISIS are, fundamentally, less scary than an ISIS that's centrally planning and launching attacks from a continent away. ISIS has a lot more money at its command, and far more experience, than individual attackers. That means it could pull off bigger attacks.

This became a major conversation topic after November's Paris attacks, as it wasn't obvious whether those attacks were planned by ISIS or inspired by the group (the evidence now suggests at least some central planning). I asked Will McCants, the director of the Brookings Institution's Project on US Relations With the Islamic World and the author of The ISIS Apocalypse, about it just after those attacks. He said that ISIS centrally planning attacks is definitely scarier.

"ISIS is a state that has millions of dollars that it can spend on these kinds of operations," He said. "We're talking about an actual government that has money to put behind plots and has very motivated people."

That said, it's plenty scary if the San Bernardino attacks were merely inspired by ISIS. It's harder for law enforcement to track people who are operating independently of ISIS home base, so if those kinds of independent or "lone wolf" attacks become more common and sophisticated it could become a serious problem.

"If [ISIS Central isn't] even trying to coordinate this kind of stuff, and their affiliates or fanboys can do it on their own, it's quite troubling," McCants told me after Paris.

One final word of caution: Even if Malik pledged allegiance to ISIS before the attack, it doesn't mean that ISIS is the key reason she and her husband decided to slaughter 14 innocent people. It could be that Farook had other reasons, as a result of his employment, to be angry at his former co-workers — and he and his wife latched onto ISIS ideology as a justification for something they wanted to do already.

Radicalization is a very complicated process; ideology matters, but so do personal grievances. "No one disputes the importance of factors other than ideology in the process of radicalization," Peter Neumann, the director of the International Centre for the Study of Radicalisation and Political Violence, wrote in a 2013 paper. It's just very hard to figure out the motivations behind violent attacks, especially with limited public information.

Bottom line: Malik's pledge to ISIS, if real, suggests that ISIS may have played some role in motivating the San Bernardino shooting. But it's far from obvious what that role might have been.