Well, the feds can't be the only ones wondering about the cause of today's massive cellular-system failure. All three major networks reported outages at roughly the same time, and although there were areas of uninterrupted service, the failures seems to be widespread. Having all three networks seemingly impacted suggested something other than a single point of failure or error. 

Unlike the rest of us, federal agencies can do something about their own curiosity. According to ABC News, the feds have started investigating whether a cyberattack took place:

A network disruption is affecting AT&T customers in the U.S. Thursday, prompting federal agencies to investigate whether the outage was caused by a cyberattack.

In a statement to ABC News, the company confirmed the outage and advised customers to make calls over Wi-Fi. ...

Two sources briefed on the situation told ABC News that the FBI and Department of Homeland Security (DHS), among other agencies, are now urgently investigating to determine whether the AT&T outages are the result of a cyberattack or a hack, or simply some sort of technical malfunction.

At least as of noon today, the agency with primary jurisdiction hasn't found any evidence of malice ... yet:

As of 5:00 a.m. ET, the U.S. Cybersecurity and Infrastructure Security Agency (CISA) reported, according to a confidential memo obtained by ABC News, that “the cause of the outage is unknown and there are no indications of malicious activity.” CISA is an agency within DHS tasked with monitoring cyber threats.

And was this really a multi-network outage? News orgs reported it that way all morning, but a commenter on our earlier Headline entry was skeptical. He thought it might just be AT&T, and that issues with other networks might be limited to calling AT&T's customers. That turned out to be correct:

Verizon and T-Mobile both told ABC News that their respective networks are not experiencing outages but customers may experience difficulty when contacting individuals affected by outages at other providers.

Of course, that doesn't eliminate the possibility of a cyberattack, but it also allows for a simpler explanation -- a single point of failure at AT&T. It also precludes other more exotic explanations, like ... a Russian nuke:

Theories about Thursday massive disruption swirled on social media, with blame for the widespread outage leveled at US foes Russia and China, as well as aliens, solar flares and even Netflix — which some suggested created the havoc to promote an apocalyptic film with an eerily similar tech blackout. ...

The most credible-sounding scenario focused on the sun, which emitted two solar flares on Wednesday evening and Thursday morning.

Those potential causes wouldn't have just impacted AT&T, though, nor just cell-phone service. NOAA, which sent out a warning about the flares earlier, shot down that hypothesis, telling the NY Post that it was just a coincidence.  

The FBI and CISA need to determine which it is, so their curiosity is welcome. As of now, it appears that AT&T has restored most of its service, so they've either found the problem or have implemented a work-around. Stay tuned.