The Baltimore 911 system crashed for at least an hour Tuesday night, leaving police and firefighters unable to receive calls to the emergency phone line, the city's Office of Emergency Management and firefighters announced.
The system went down around 8:20 p.m., city officials said. Mayor Stephanie Rawlings-Blake announced around 10:15 p.m. that it was restored.
The crash was suspected to be caused by a problem with Verizon, said Robert Maloney, director of the Mayor's Office of Emergency Management.
Verizon maintains equipment and networks for the city, spokesman John Johnson said. But he was unsure Tuesday night whether the equipment or network failed.
"We are investigating the cause and have a technician on site," he said.
Residents were urged to call 311 with emergencies.
When it was down, calls to 911 were immediately answered with a recorded message, "Baltimore City emergency center, all operators are busy. Your call will be answered in turn. Please do not hang up."
The city's 911 center is the busiest in the state, sometimes handling 4,000 calls a day.
In October, Rawlings-Blake put the fire and police departments in charge of the 911 system. The 911 system had been criticized by residents who said they experienced dropped calls, no answers, and busy signals when trying to report an emergency.
After the system was restored Tuesday, police said the lines would be closely monitored through the night.
Baltimore Sun reporters Yvonne Wenger and Kevin Rector contributed to this report.