What caused the Verizon outage yesterday? What we know.

On Monday, tens of thousands of Americans were hit hard by a widespread Verizon outage that left the network offline for several hours. Although the issue has now been resolved, frustration remains for healthcare workers, students and countless others who rely on their cell phone service.

Now the big question: What caused it?

According to telecom experts who spoke to CNN, these outages typically stem from one of three culprits: overloaded networks in urban centers, botched software updates (like AT&T's) or technical glitches. While these blanket explanations cover some basics, they don't say exactly what went wrong this time.

It's not the first time this year that phone users have experienced a major outage. AT&T experienced a similar collapse in February.

To be clear, at this point there is no official word on what caused the Verizon outage. The FCC has launched an investigation, but Verizon remained mostly silent, offering only a brief apology, acknowledging the disruption and saying the problem would be resolved.

Even though the network appears to be back online, the exact cause of the crash remains a mystery for now.

Mashable reached out to Verizon's communications team for comment, but they have not yet responded.

This is a developing story. It will be updated as more details become available.