WASHINGTON (Reuters) - The U.S. Federal Communications Commission said on Thursday it will investigate why AT&T wireless subscribers in several states could not make emergency 911 calls late Wednesday.

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The FCC has previously fined carriers that had 911 outages that it deemed preventable and required steps to prevent further outages.

A person briefed on the matter said a software glitch apparently caused the outage, which lasted a few hours and spanned at least 10 states.

“Every call to 911 must go through,” FCC Chairman Ajit Pai said in a statement. “I have directed commission staff to track down the root cause of this outage.”

Pai said he spoke to AT&T Chief Executive Officer Randall Stephenson about the matter.

AT&T takes its 911 obligations very seriously, said spokesman Mike Balmoris.

“We are taking steps to prevent this from happening again and will be sharing additional information with the FCC,” Balmoris said.

The National Emergency Number Association, a nonprofit group, said problem shows the need to upgrade technology at 911 centers so calls can be redirected to other regions.

Several carriers agreed to settlements after an April 2014 outage affected 11 million telephone users.

Verizon Communications Inc agreed to a $3.4 million fine after a six-hour 911 outage in April 2014 that affected about 750,000 wireless consumers in nine California counties.

CenturyLink Inc agreed to a $16 million settlement in the April 2014 outage and Intrado Communications, a unit of West Corp, agreed to pay a $1.4 million fine.

The FCC said the outages at the carriers in April 2014 resulted in 6,600 missed 911 calls about domestic violence, assault, motor vehicle accidents, a heart attack, an overdose, and an intruder breaking into a residence.

The April 2014 outage was the result of a preventable software coding error at a call management center in Colorado, the FCC said.

In 2015, T Mobile US Inc agreed to a $17.5 million settlement after two 911 service outages nationwide in August 2014. The separate but related outages lasted approximately three hours and affected almost all of T-Mobile’s then 50 million customers.