Additional charges are likely in the criminal case against two longtime associates of President Trump’s personal lawyer, Rudolph W. Giuliani, a federal prosecutor said Monday.

The associates, Lev Parnas and Igor Fruman, were arrested in October and charged with concealing the source of political donations in order to advance their own business interests and the political interests of Ukrainian government officials. They have pleaded not guilty to four criminal counts related to violations of campaign finance laws.

At a hearing Monday in Manhattan, a federal judge asked prosecutors if they intended to update their indictment with new charges. Douglas S. Zolkind, an assistant United States attorney, responded that the investigation was not over. “We think a superseding indictment is likely, but no decision has been made,” Mr. Zolkind said.

Later in the hearing, when a lawyer for Mr. Parnas asked that house arrest conditions be loosened for his client, Mr. Zolkind said Mr. Parnas was still under investigation for additional crimes.