Verizon's nationwide outage that left thousands of people without service on Monday has sparked fears that the disruption was caused by a major cyberattack.

Customers were left in the dark for nearly 12 hours after they were stuck on SOS mode from 9:30am ET until 7:30pm ET.

While Verizon has not revealed the cause, experts told DailyMail.com that a cyberattack by Russia or China 'is plausible.'

The CEO of a leading global cybersecurity firm explained that hackers could've prompted Verizon's online routers to release a malicious signal to cell towers, disrupting communications for thousands of people.

Kyle Hanslovan, the CEO of Huntress - a leading global cybersecurity firm told DailyMail.com: 'This could be a call to action that if the [Verizon outage] was a hijacking, what are companies doing to prevent these kinds of attacks from happening.'

Hanslovan noted that 'it isn't clear whether the Verizon Wireless outage is the result of a mistake or something more nefarious,' adding: 'However, the likelihood this was a purposeful cyberattack seems just as plausible.' 

At least 100,000 people were impacted by the Verizon outage, with customers living in the Midwest including Illinois, Wisconsin and Indiana being hardest hit. 

Mobile phones switched to SOS mode, limiting communications to only emergency calls.   

Verizon hasn't confirmed what caused yesterday's outage, but Dave Hatter, the director of business growth at Intrust IT told DailyMail.com: 'The fact that Verizon has not indicated what cause the problem or how they will ensure that it will not happen again has caused some to speculate that it could be a cyberattack. 

'That said, they may be tight lipped about it to ensure that information that could lead to a cyberattack is not released.'

The Federal Bureau of Investigation (FBI) has warned that cybercriminals in countries like China target critical infrastructure to 'physically wreak havoc' or disrupt services at any time they choose.

Hanslovan speculated that the Verizon outage could have been caused by hackers accessing some of the company's online routers and redirecting signals to and from Verizon's cell towers to disrupt cell service throughout the country.

If a cybercriminal redirected the signals sent from those towers, it's possible it would have caused people's phones to stop working.

'There's a history in China where attacks have worked like this. Where entire worldwide traffic was misdirected,' Hanslovan said.

'Sometimes this happens by complete accident, maybe the person was misinformed and redirected the traffic to the wrong place.'

However, Rob DeCicco who previously worked for the National Security Agency (NSA) told DailyMail.com that it's likely the outage was caused by Verizon focusing on 'rolling out 5G in other regions and a lack of personnel monitoring performance.'

The Federal Communications Commission (FCC) is currently looking into what caused phones to switch to SOS mode.

'At the moment there is no information pointing to one either from Verizon or from the multiple ransomware groups,' Carlos Perez, a spokesperson for the cybersecurity company, TrustedSec, told DailyMail.com.

'It does not mean it is not [a cyberattack], but matters like this take time to identify a root cause,' he continued.

'If it is a cyberattack, [Verizon is] obligated to report it as part of their SEC filling as a material incident to the agency.'

AT&T denied a cyberattack when it's systems were breached in 2021, but was forced to acknowledge earlier this year that hackers had obtained significant amount of customer data including text records, location data and phone numbers.

Fortunately for customers, Hanslovan said their data was likely protected thanks to encryption software used internally by companies that used to safeguard people's information.

Perez added that he doesn't believe people need to take steps to protect their data 'given that we do not know the scope and systems involved in the outage.

'Once Verizon puts more data of what was affected we should know more but it is safe to say that customer should still use a PIN to allow porting of their number or changes to their e-Sims as a basic precaution, outage or not,' he said.

The bigger question will be what companies are doing to protect people from future mistakes and potential cyberattacks.

There is an extreme amount of effort that goes into maintaining cell phone infrastructure worldwide, according to TrustedSec's advisory solutions director, Alex Hamerstone.

'In fact, I think many people would end up being surprised that things work as often as they do,' he previously told DailyMail.com.  

Maintaining the cell phone infrastructure is a two-tiered process, Hamerstone said, because the workers need to keep the existing system functioning while also continuously improving, updating and expanding the systems, networks and equipment.

This also includes improving the infrastructure's resilience to security threats.

He clarified that a cyberattack has the possibility to cause a cell phone outage at any time, although 'it is not generally the most likely.'  

But, 'regardless of the reason, democratic countries and telecoms worldwide need to ask themselves: 'Are we predictively doing enough to encrypt traffic, modernize the backbone of the internet and warn those at risk?' Hanslovan said.

'In today's era, it's not 'if' a cyberattack like this will happen, but 'when' it will happen again.' 

DailyMail.com has reached out to Verizon for comment.