AT&T said that it has fully restored wireless service to all of its customers after an outage on Thursday morning that disrupted phone service nationwide.

"We have restored wireless service to all our affected customers," AT&T said in a statement to Spectrum News. "We sincerely apologize to them. Keeping our customers connected remains our top priority, and we are taking steps to ensure our customers do not experience this again in the future."

Users of several major cellular carriers, including AT&T, Verizon and T-Mobile, reported outages on Thursday morning, according to Downdetector, which tracks internet, communication and other service outages.

AT&T said in a statement that after an initial review they conducted, the company believes that the interruption "was caused by the application and execution of an incorrect process used as we were expanding our network, not a cyber attack."

Per Downdetector, users began to report issues after 3:30 a.m. ET. Reports peaked at at around 4:30-5:30 a.m. ET, depending on the carrier in question. 

Areas reporting issues included New York, Los Angeles, Dallas and Houston, Chicago and Atlanta. Some municipalities across the country also reported issues with contacting emergency services, including 911.

The emergency services outage prompted at least one law enforcement organization to urge people not to test if 911 was working.

“Many 911 centers in the state are getting flooded w/ calls from people trying to see if 911 works from their cell phone. Please do not do this,” Massachusetts State Police wrote on X, formerly Twitter. “If you can successfully place a non-emergency call to another number via your cell service then your 911 service will also work.”

AT&T had more than 64,000 outages on Thursday morning. The carrier has more than 240 million subscribers, more than any other company in the U.S.

Cricket Wireless had more than 13,000, the outage tracking website said Thursday. Verizon had more than 4,000 outages and T-Mobile had more than 1,900 outages reported. Boost Mobile had about 700 outages.

In a statement to Spectrum News, Verizon said its network is "operating normally."

"Some customers experienced issues this morning when calling or texting with customers served by another carrier," the carrier said. "We are continuing to monitor the situation."

Likewise, T-Mobile said that it did not experience an outage.

“Our network is operating normally," the company said in a statement to Spectrum News. "Down Detector is likely reflecting challenges our customers were having attempting to connect to users on other networks."

At a briefing on Thursday afternoon, White House national security council spokesperson John Kirby said that the Department of Homeland Security and the FBI are investigating the incident, but AT&T has told officials there's no reason to think it's a cybersecurity incident. The FCC has also been in touch with AT&T about the incident.

When asked if any government communications were impacted, Kirby said that "there was some impact to [the Commerce Department], but I don’t know the extent of that. I don’t think it was crippling." 

NOTE: Spectrum News is owned by parent company Charter Communications, which owns and operates Spectrum Mobile. 

The Associated Press contributed to this report.