The two priests were found dead Monday, hours after being abducted in Poza Rica, Veracruz.
Sidronio Casarrubias is an alleged leader of the Guerreros Unidos gang, which the government alleges killed the students in September 2014 and incinerated the bodies.
The gunman wrote the letters "RB" and other markings, but the meaning was unclear.
The first suspect charged in the kidnapping of Alan Pulido has given a statement to prosecutors confessing to the crime.
The mother could lose the custody of her children.
Nobody has presented a formal accusation against the subject, according to the authorities.
Calling for information, they issued a photograph of a man, dressed in a white shirt and jacket and wearing a dark hat as he pushed a luggage trolley through the airport.
The young man claims to be a member of the "Los Rojos" cartel.
Police and firefighters say hospitals treated at least 35 people from several states, Mexico and Canada. Six people remained hospitalized Wednesday, including two in critical condition.
Authorities say Robert Lewis Dear killed three people, including a police officer.