The FCC on Wednesday fired off letters to Tele-Communications Inc. and 15 individual cable TV systems nationwide asking them to justify changes in cable rates and services undertaken in anticipation of federal regulation of the industry.

The FCC “letters of inquiry with investigatory overtones” represent a get-tough approach by the agency in dealing with allegations that cable operators illegally jacked up rates after passage of the 1992 Cable Act.

The letters were mailed one day after it was disclosed that a top executive with TCI, the nation’s largest cable company, wrote a memo Aug. 20 encouraging TCI general managers to begin charging customers for upgrades and VCR hookups.

Acting FCC chairman James Quello said Wednesday, “I plan on ensuring that the TCI memo and the other informal complaints are investigated fully, and appropriate action taken where necessary.”

Quello said the investigation could lead to sanctions against individual cable operators or a change in FCC rules to ensure that consumers are charged “reasonable rates.”

The FCC inquiry comes in response to complaints lodged by mayors, local officials and consumers.

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