He said the seminary was eager to cooperate with law enforcement authorities in the investigation.

Mr. Wright began his search on the Tijuana section of Craigslist, where he posted an advertisement in 2014 looking for a child to adopt, according to a criminal complaint filed on Friday.

He traveled to Mexico that July after someone responded to the ad, offering him a child. But the response was part of a swindle: He met the person in a hotel and paid an “adoption fee,” but the person never delivered. Mr. Wright returned to the United States alone.

The person in Tijuana was central to the investigation against Mr. Wright, according to law enforcement officials, who declined to provide details about that person’s identity.

The seminarian and the con artist encountered each other on Craigslist again a year later, when Mr. Wright posted an ad looking for a tour guide in Tijuana. The person who had defrauded him, using a new email address, sent him a response.

During their correspondence, Mr. Wright said he was traveling to Mexico, in part to find a woman to marry and a child to adopt. Eventually, he said, he was interested in sexually assaulting female infants.

The person contacted the National Center for Missing and Exploited Children in Alexandria, Va., which forwarded the tip to the Department of Homeland Security.

The person handed control of an email address to Homeland Security on Dec. 3, 2015, and agents continued the conversation with Mr. Wright. He wrote in graphic detail about his plans to adopt or “rent” female infants to sexually assault, according to the criminal complaint.