A federal grand jury is investigating an unauthorized attempt to gain access to the Federal Reserve Board's computer files of sensitive financial information, sources reported yesterday.

Sources said the investigation involves Theode C. Langevin of Laurel, a former Federal Reserve Board aide who left the board recently to join E.F. Hutton & Co., a Wall Street securities firm.

Thomas Rae, general counsel of E.F. Hutton, said Langevin was "hired to be a 'Fed watcher' and economist, but "came and was gone before he actually performed those duties."

A 'Fed watcher' on Wall Street follows Federal Reserve actions closely to advise on the board's credit policies, which have powerful impacts on interest rates and securities prices.

Rae said Langevin was asked to leave E.F. Hutton, but said he couldn't discuss the reasons and would not comment on the reported investigation. Langevin, reached at home, said he left voluntarily. Langevin said he has hired an attorney but would not discuss reports that he is the subject of an investigation.

Sources said that, within the past month, someone attempted to gain access by telephone to the Fed's computer data files that record sensitive information on the changes in the nation's money supply -- a key factor determining Fed actions.

Authorized Fed employes are able to tie into the computer by using a numerical code that is transmitted by telephone. When employes leave the Fed, their code numbers are cancelled, sources indicated.

The unauthorized attempt, involving a cancelled code, was blocked by the computer, and the telephone call was traced to a New York City address, sources said.

A Federal Reserve spokesman said there is an investigation underway, but would not give details and referred questions to the U.S. Attorney's Office.