Mr. Lelling said at the time that the investigation “remains active” and suggested that more people would be charged. But the investigation has appeared to move slowly since then, and Ms. Sui — a Chinese citizen residing in Canada — is only the second additional defendant to be named. Mr. Lelling’s office said the indictment unsealed on Tuesday had been returned by the grand jury in March.

At the center of the case is William Singer, an independent college admissions consultant whom prosecutors have described as the mastermind of the scheme. Mr. Singer has pleaded guilty to racketeering and other charges and is cooperating with prosecutors.

The indictment against Ms. Sui says she conspired with Mr. Singer and a new person in the case, “Recruiter 1,” who is based in Sarasota, Fla., and owned and operated a service that matched high school tennis players with college coaches. It was not clear whether Recruiter 1 had connected Ms. Sui with Mr. Singer.

According to the indictment, Mr. Singer spoke to Recruiter 1 by phone in early August 2018, and listed the price it would cost to secure Ms. Sui’s son’s admission to various schools, including U.C.L.A., through bribery.

The next month, Ms. Sui sent Recruiter 1 photographs of her son playing tennis, which he forwarded to Mr. Singer, the indictment said. But Mr. Singer had apparently decided to portray Ms. Sui’s son as a soccer player, not a tennis player. He sent a fake soccer résumé, which included a picture of another person playing soccer, to Jorge Salcedo, the men’s soccer coach at U.C.L.A.