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.

“Some of our customers are experiencing wireless service interruptions this morning. We are working urgently to restore service to them. We encourage the use of Wi-Fi calling until service is restored," an AT&T spokesperson said.

Later Thursday morning, AT&T issued an update saying 75% of its network had been restored.

"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," the company said.

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.

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.

Several police departments and municipalities have warned local residents of what they've described as a nationwide outage. In turn, officials have urged callers to contact emergency services by alternative means.

"There is a nationwide AT&T outage that is preventing wireless customers from making and receiving any phone calls (including to 9-1-1)," the Charlotte-Mecklenburg Police Department, which serves the Charlotte, North Carolina area, said in a post on X.

The county government in Fairfax, Virginia released a similar warning.

"There is a nationwide AT&T outage that is preventing wireless customers from making and receiving any phone calls (including to 9-1-1)," the Fairfax County Government said on X. "Try calling from a landline or ask a friend or family member to call 9-1-1 on your behalf."

In response to an earlier request from ABC News, CISA said they had no comment on the outages.

AT&T serves more than 100 million customers across mobile and broadband services, according to the company's website.

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.

"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," a Verizon spokesperson said.

 

T-Mobile similarly 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."