In an interview last month, Frank Church, chairman of the Senate committee that is investigating the CIA, issued an oblique but impassioned warning, that the technology of eavesdropping had become so highly developed that Americans might soon be left with "no place to hide". That day may have arrived. Newsweek has learned that the country's most secret intelligence operation, the national Security Agency, already possesses the computerized equipment to monitor nearly all overseas telephone calls and most domestic and international printed messages.

The agency's devices monitor thousands of telephone circuits, cable lines and the microwave transmissions that carry an increasing share of both spoken and written communications. Computers are programmed to watch for "trigger" words or phrases indicating that a message might interest intelligence analysts. When the trigger is pulled, entire messages are tape-recorded or printed out.

That kind of eavesdropping is, however, relatively simple compared with the breakthroughs that lie ahead in the field of snoopery. Already it is technically feasible to "bug" an electric typewriter by picking up its feeble electronic emissions from a remote location and then translating them into words. And some scientists believe that it may be possible in the future for remote electronic equipment to intercept and "read" human brain waves.

Where such capabilities exist, so too does the potential for abuse. It is the old story of technology rushing forward with some new wonder, before the men who supposedly control the machines have figured out how to prevent the machines from controlling them.