A cyberattack is being considered as the possible reason for this morning's AT&T nationwide outage

UPDATE:AT&T says that all service has been restored.

U.S. federal agencies are investigating what caused a major outage for AT&T subscribers this morning. With AT&T's network going down, Mobile Virtual Network Operators or MVNOs that rely on the AT&T network, such as Cricket, also suffered outages. Verizon and T-Mobile claim that their networks never had any issues and the problems that their subscribers had were due to their inability to connect to an AT&T customer.

As an aside, analysis of Google Search data discovered a whopping 567% increase in online searches for 'Cancel ATT' on Thursday. In an online statement, AT&T said this morning, "Some of our customers are experiencing wireless service interruptions this morning. Our network teams took immediate action and so far three-quarters of our network has been restored. We are working as quickly as possible to restore service to remaining customers."

Two sources briefed on the situation told ABC News that the Department of Homeland Security (DHS), along with other agencies, and investigating the outage to see if it was caused by a cyberattack or a technical malfunction. ABC News also obtained a confidential memo that cited the U.S. Cybersecurity and Infrastructure Security Agency (CISA). CISA stated that "the cause of the outage is unknown and there are no indications of malicious activity." CISA is the agency inside the DHS responsible for monitoring cyber threats.

CISA had no additional comments about the outages in response to a request from ABC News.

Local police departments are warning residents about what they call a "nationwide outage" and are telling them to contact emergency services, if needed, by using alternative means. For example, in a tweet, the Charlotte-Mecklenburg Police Department, which handles the region that includes Charlotte, North Carolina, wrote, "CMPD is aware of a nationwide outage impacting AT&T cell phone customers this morning. Customers were briefly unable to contact 9-1-1. There are no disruptions to our call center’s ability to receive 9-1-1 calls. Service should be returning shortly."

AT&T has approximately 100 million mobile and broadband customers who were affected by this morning's outage. Verizon told ABC News, "Verizon's network is operating normally. Some customers experienced issues this morning when calling or texting with customers served by another carrier. We are continuing to monitor the situation." T-Mobile told ABC News, "We did not experience an outage. Our network is operating normally. Down Detector is likely reflecting challenges our customers were having attempting to connect to users on other networks."