A widespread and hours-long outage dinged the reputation and the stock price of incumbent telecom company AT&T (T -0.47%) on Thursday. Like the outage itself the damage was contained; still, investors traded out of the stock, and it closed the day more than 2% lower. That compared unfavorably to the trajectory of the S&P 500 index, which rose at roughly the same percentage rate.

Outages remain a mystery

AT&T's wireless outage began around 3:30 a.m. ET on Thursday and affected people in cities throughout the U.S. -- Los Angeles, New York, and Atlanta among them. It lasted for several hours. According to reporting from The New York Times, at the incident's peak approximately 70,000 reports were logged on the Downdetector.com outage tracking website.

The two other American telecom incumbents, Verizon Communications and T-Mobile US, also reported service interruptions, but at lower volumes. Verizon received 3,000 reports at a certain point on Thursday, and T-Mobile about 1,500.

All three companies said they were operating normally by mid-afternoon. AT&T's latest update on the matter, posted at 3:30 p.m. ET, said that all affected customers had had their services restored. "We apologize to them," the company wrote. "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."

It did not elaborate on those measures being taken.

Currently under investigation

The reason or reasons for the outages remain unclear. The federal government has ruled out a hacking incident, the Times wrote, and several of its agencies are investigating the matter in collaboration with the affected companies. In addition to the Federal Communications Commission (FCC), these agencies include the Department of Homeland Security and the FBI.