A federal grand jury yesterday indicted the alleged "mastermind" of a massive fraud scheme in which hundreds of prospective purchasers of cars and other property here had their adverse credit histories laundered for a fee.

The grand jury is continuing its investigation of the alleged fraudy ring, according to Assistant U.S. Attorney Raymond Banoun of the fraud division. Twelve persons already have pleaded guilty in federal and local courts here as result of the probe.

Identified by investigators as the key figure indicated yesterday was Eddie Lee Alston, 44, of 1821 L. St. NW. who was described as a free-lance collection agent. Also charged with conspiracy to commit fraud, in a separate case concerning the same scheme, was Stanley William Holmes, 45, of 5415 CHevy Chase Pkwy., Chevy Case, a car salesman at Rosenthal Chevrolet.

The indictments, which said the alleged scheme had been in operation for more than two years, also listed 10 area car dealerships were salesmen would arrange to have bad credit records altered for fees ranging up to $800. No criminal charges were placed against the car dealerships.

According to the charges, Alston would recruit automobile salesmen whose prospective customers had unfavorable credit ratings. Alston and others involved in the alleged scheme would then arrange to have the credit histories altered for a fee through an "insider" at The Credit Bureau Inc., whom Alston also had recruited, the charges continued.

The money paid for the alteration - ranging from $200 to $800 per customer - would then be divided among the various members of the alleged conspiracy, the indictment said.

Banoun said the alleged scheme "exploited the financial distress of persons who had poor credit" and defrauded banks, private lending institutions, department stores and credit companies who relied on supposedly accurate credit ratings.

Alston was charged in a 39-count indictment with conspiracy, mail fraud, wire fraud and false pretenses. Holmes was charged with conspiracy and mail fraud in a three-count indictment.

Among the alleged co-conspirators named in yesterday's indictments were several of the persons - including Credit Bureau "insiders" and automobile salesmen - who already have pleaded guilty to charges growing out of the same fraud scheme.

The automobile dealerships named in the indictment whose salesmen allegedly were aware of or participated in the alleged scheme included Senate Dodge, Inc.; Dave Pyles Lincoln-Mercury, Inc.; Seldel Chevrolet, Inc.; Safford Lincoln-Mercury, Inc.; Lustine Chevrolet, Inc.; Northeast Ford, Inc.; Ourisman Chevrolet, Inc.; Blank Pontiac, Inc.; Sheehy Ford, Inc.; and Loving Chevrolet, Inc.