Currently, police and transit officers use separate radio networks and have to pass messages to each other through central dispatchers. The integrated network was first conceived in 1995, when the transit police force was merged into the NYPD.

The NYPD "said from the start that the issue of substantial interference would have to be fixed before the system would be accepted by us," Paul Browne, deputy police commissioner for public information, said in the same e-mail in which Kelly was quoted. "The interference experienced during the testing indicated that certain transmissions were inaudible or not understandable. It's a safety issue that we're working with the MTA to resolve."

The MTA's New York City Transit division concurs that distortion problems exist where the radio signals from inside and outside the subway meet. Joseph L. Yurman, principal communications engineer at the NYCT, said in an interview that because the subways have been built over many decades at relatively shallow depths, there is "insufficient isolation" between the signals above and below the ground.

The amount of distortion is "subjective" and varies from location to location, Yurman said. "It's a physics problem, and there's no software you can buy and -- poof -- it works. It's very, very complicated, and there are a lot of variables."

But, Yurman added, the NYCT has been working with several companies to pilot digital signal processing technology that could solve the problems. The new technology has eliminated the distortion in tests, he said, and the NYCT hopes to roll it out in one of the many subsystems that make up the overall network by midyear or year's end at the latest. "We're working as fast as we can," he said.