MEXICO CITY — The federal police captured a reputed founder and senior leader of the Zetas drug gang who may be connected to the killing of an American agent earlier this year, Mexico’s security ministry said Monday.

In a statement, Mexican officials described the suspect, Jesús Enrique Rejón Aguilar, as the third most powerful leader of the Zetas, who have become a dreaded source of violence, especially along Mexico’s eastern coast, where they are fighting for territory with the Gulf Cartel.

Experts said the arrest was a sign that the administration of President Felipe Calderón had refocused its efforts against the criminal group, but Eric L. Olson, a senior associate at the Mexico Institute of the Woodrow Wilson International Center for Scholars, said that by itself, the arrest should not be seen as a triumph.