The Middlesex district attorney's office Monday dismissed armed robbery charges against a suspect in last year's Fogg coin theft because there was insufficient evidence to support an indictment.

The dismissal closed the case of the only Fogg suspect who, to date, has been prosecuted by the state of Massachusetts.

Robert J. Barker, assistant district attorney investigating the case, said yesterday that "the evidence presently in the possession of the commonwealth does not properly suggest the indictment" of Leonard Piazza, 45, of Lynn. Barker added that his office will continue to work in conjunction with federal authorities to find the "right" man.

Cambridge police arrested Piazza on December 13, 1973, after two eyewitnesses identified him as a participant in the $2 million rare coin theft that took place 11 days before.

Piazza, who has a long criminal record, was subsequently convicted on unrelated arson charges and is currently serving a term at Walpole.

Daniel Steiner '54, general counsel to the University, said yesterday that Piazza was apparently not connected to the Fogg robbery. "The innocent man was let loose and this is encouraging," he said.

Steiner said that the FBI is still working hard on recovering the balance of the stolen coins. About 40 per cent of the coins were found on November 5 in Lincoln, R.I. Another 10 per cent of the coins were recovered three days later in Canada.

James L. Newpher, special agent in charge of the Boston field office of the FBI, said yesterday that most of the investigation is concentrated in the New England area.

Newpher attributed Friday's dismissal of federal charges of conspiracy to transport stolen goods across state lines against two other suspects to "an interpretation of the law." He added they still may be indicted.