HOUSTON - The Federal Communications Commission is investigating why some AT&T wireless customers were unable to dial 911. The glitch appears to be fixed but caused major concerns in Houston and cities nationwide.

"No one knows for sure exactly what happened, but we do know some people were not able to reach 911 for a period of up to 5 hours ," said Trey Forgety of the National Emergency Number Association.

The Greater Harris County 911 Network says it is communicating with AT&T to determine what impact the outage had on calls to the Houston area.

AT&T resolves outage preventing customers from making 911 calls

The outage prompted Houston Police and HPD Chief Art Acevedo to take to Twitter, telling emergency callers to contact the city's non-emergency dispatch number.

"We apologize to those affected," said AT&T in a statement to media. "We take our obligations to customers very seriously and will be sharing additional information with the FCC."

"How 911 calls are delivered can be very complex," said Forgety. "It will take a while for the FCC to sort through an investigation."

The National Emergency Number Association suggests residents keep the non-emergency 10-digit phone numbers for local police, fire or EMS saved in a mobile phone to use as a back up.

Residents in Houston, Harris County and Fort Bend can send text messages to 911 if they are unable to get through on a voice call.