CenturyLink was slapped with a $16 million fine this week for a 911 failure in Minnesota and other states a year ago.

For more than six hours in April 2014, 11 million CenturyLink, Intrado Communications and Verizon customers in seven states could not reach emergency call centers, according to the Federal Communications Commission, which recently concluded an investigation of the event.

Intrado was fined $1.4 million and Verizon was fined $3.4 million last month.

The FCC issued the penalties because the failure did not stem from a natural disaster or other unforeseeable catastrophe. It resulted in 6,600 missed 911 calls, including emergency calls about domestic violence, assault, auto accidents, a heart attack, an overdose and an intruder breaking into a home.

"Americans need to be confident that the service they use to reach first responders is reliable and accessible in their time of need," FCC Chairman Tom Wheeler said in a statement. "Providers have a responsibility to ensure that Americans can use 911 to call for help any time. When a company fails to live up to its obligations, it will be held accountable."

The FCC concluded that the failure was preventable if the providers had used basic safeguards. CenturyLink and Intrado failed to provide timely notifications to the affected emergency call centers, according to the FCC. The CenturyLink outage was in Minnesota, Washington and North Carolina. Intrado's was in Florida, South Carolina and Pennsylvania.

The settlement for CenturyLink, Minnesota's largest phone carrier, is the largest 911-related fine ever assessed by the FCC.

"CenturyLink takes 911 reliability very seriously," the company said in a statement. "This outage was caused by a third-party vendor's equipment failure that had never occurred before, and we worked with the vendor to implement measures to ensure that this type of failure will not happen again. CenturyLink values customer safety and makes reliable 911 communications a top priority."

CenturyLink and Intrado agreed with the FCC to do a better job avoiding and recovering from such failures in the future.

Last month, the FCC settled with Verizon for $3.4 million in connection with the same failure. Verizon served emergency call centers in California and agreed to the same compliance terms as CenturyLink and Intrado.