He also alluded to evidence of additional families — not charged at this point — who he said initially pursued the admissions scheme for their children before backing out of it, saying, “There are a lot of kids or parents who at the beginning plotted with Singer and then withdrew for a variety of reasons.”

Of the parents, coaches and others already charged in the case, 20 people, including the actress Felicity Huffman, have pleaded guilty or agreed to do so. Mr. Singer, who cooperated with the government in its investigation, has pleaded guilty to racketeering and other charges. Thirty others, including the actress Lori Loughlin and her husband, the designer Mossimo Giannulli, have pleaded not guilty.

[One scandal, two actresses take diverging paths.]

It’s unclear for the moment how many additional parents have newly been told by the government that they are under investigation, but some of the parents are considering pleading guilty, while others believe they have done nothing illegal, according to two defense lawyers who represent some of them.

Among a contingent of white-collar defense lawyers in Los Angeles and Boston, seemingly everyone either already has a client in the case or is considering taking one. The lawyers who are new to the case — whose clients have not been charged but are waiting and worrying — are calling their colleagues who are already immersed in it for orientation, asking for their assessments of Mr. Rosen, and other basic information, like what kind of plea deals the government has offered and whether defendants have received discovery yet.

The prosecutors’ steps to investigate additional parents already have moved beyond phone calls and target letters: They have issued subpoenas seeking banking records, information from accountants and phone logs, according to two defense lawyers representing families who are now targets.

All the while, there is a different group of Southern California parents — people who had hired Mr. Singer but haven’t heard from the prosecutors at all — who are worried enough to have hired lawyers anyway. Several of them are weighing whether to volunteer themselves to the authorities, in the hopes of lenient treatment, or whether to lie low and hope their circumstances do not draw notice.