Baltimore emergency management officials scrambled to figure out how to notify the public that the city's 911 system was out of service Tuesday night, according to internal emails obtained by The Baltimore Sun .

City officials quickly decided to use the 311 call center as an alternative but debated for at least a half hour how to let the public know about the problems, according to emails forwarded to a reporter by Robert Maloney, director of the office.

At a news conference, Police Commissioner Kevin Davis defended the city's response to the problem, which lasted nearly two hours.

Police and fire officials said they acted quickly after receiving notifications the system was down. They gathered in a room at police headquarters and 911 operators began taking 311 calls. Officials could not say how many 911 calls were diverted to 311 or whether response times were delayed.

Officials at Verizon — the service provider for the city's 911 system — said the phone company received an automated alert at 7:48 p.m. reporting that 911 calls were failing. Verizon spokesman John O'Malley said the company eventually determined that emergency calls were mistakenly routed to an empty back-up call center in the city.

"Technically the calls were coming in, but they were getting routed to a location where no one was there to pick up," O'Malley said.

On Wednesday, Verizon was still trying to determine what caused its equipment to send calls to the wrong center. The 311 system works on technology similar to 911's but was not affected. O'Malley said that it's unusual for a jurisdiction to have an entire back-up center and that Verizon had not experienced a similar problem in the past.