A lawyer pleaded guilty Monday to impersonating representatives of both a hedge fund and a pension fund in order to assist disgraced ex-attorney Marc S. Dreier in selling a phony promissory note.

The surprise plea and the lawyer’s agreement to cooperate with prosecutors raised the possibility there might be more arrests in the investigation of Dreier, the former sole equity partner of 250-lawyer Dreier LLP who is serving a 20-year sentence for peddling hundreds of millions of dollars in bogus notes to investors.