WASHINGTON — The FBI disclosed new details about its secretive technology for tracing telephone calls and recording conversations during criminal, terrorism, and espionage investigations, custom-made tools it has developed quietly for a decade.

Documents released under the Freedom of Information Act show the government has come a long way since the days of alligator clips and reel-to-reel tape recorders when it comes to its surveillance techniques.

In hundreds of heavily censored pages, the FBI described in unprecedented detail a sophisticated surveillance system known as the Digital Collection System Network. It includes programs to record information about telephone calls — such as the number called and the duration of the call — made by surveillance targets and another program called Digital Storm to record conversations.

Many of the documents were marked “for official use only.”

Some of the FBI files describe the security risks that outsiders might gain access to the bureau’s eavesdropping tools. The FBI said part of its surveillance system is tightly guarded, physically against intruders and electronically against hackers.

Steve Bellovin, a security expert and computer science professor at Columbia University, describes the system as “basically a control terminal” for the agency’s various surveillance taps.

He said the biggest threat would be not from hackers but from inside. “Can it be abused by rogue agents?” he asked.

In files describing the operation of the call-tracing system, known as the DCS–3000, the FBI acknowledged that rogue agents or spies represent the gravest threat. “As with most information systems, the greatest threat to the DCS–3000 would come from the inside,” the FBI said. “Since they have access to the system at various levels, users could damage, alter or erase data and destroy system hardware and software. They could also use the information gathered by it for profit by passing on the collected information or by alerting those being monitored.” “We go to great lengths to make sure the system is secure,” said Anthony DiClemente, section chief of the data acquisition and intercept section at the FBI. Access is limited, and the system itself is kept “in a locked room,” he said. The FBI was forced by a federal judge to give the files to the Electronic Frontier Foundation, a San Francisco-based civil liberties group, which requested them under the Freedom of Information Act more than one year ago.