MEXICO CITY — The chief of criminal investigations for Mexico’s attorney general resigned late Wednesday amid an internal affairs inquiry into his office’s handling of the case of 43 college students who vanished nearly two years ago.

A brief government statement said the attorney general, Arely Gómez González, had accepted the resignation of Tomás Zerón de Lucio from his post as head of criminal investigations, and “wished him success in his personal and professional projects.”

The statement did not explain the reasons for the resignation, but an official in the attorney general’s office said it was intended to repair relations with the students’ families, who have maintained relentless pressure on the government to solve the case and had demanded Mr. Zerón’s removal.

But Mr. Zerón was not leaving government entirely. Hours after his resignation, President Enrique Peña Nieto named him as the “technical secretary” of Mexico’s National Security Council. Mr. Zerón could not immediately be reached for comment.