The most prominent journalist sued recently is Carmen Aristegui, a broadcast journalist who in 2014 led a team that broke the first major story about alleged impropriety by Mr. Peña Nieto. Ms. Aristegui and her colleagues meticulously documented an arrangement under which a construction company that had been awarded lucrative government contracts built a mansion for the president’s wife, Angélica Rivera.

The news organization that employed them at the time, MVS, which depends heavily on government advertising, refused to run the story, according to Ms. Aristegui. The journalists published it independently and later chronicled the saga and its fallout in a book that came out last year. In July, the owner of MVS, Joaquín Vargas, sued Ms. Aristegui, alleging “moral damages.” That lawsuit is one of 10 filed recently against journalists that are being tracked by the Organization of American States.

“This has raised an alarm,” Edison Lanza, the special rapporteur for freedom of expression at the O.A.S., said in an interview. “It seems likely that some public officials are taking advantage of this.”

One plaintiff is Humberto Moreira, the former governor of the state of Coahuila, who filed lawsuits in July against two journalists over articles that referred to corruption allegations that have long dogged him. One of the journalists, Sergio Aguayo, has said that he suspects the case was brought to deter him from continuing to investigate a massacre in Coahuila in 2011. The governor is suing him for roughly $535,000.