A former seminary student arrested for allegedly seeking sex with infants and young girls was carrying baby clothes and a bottle in his luggage, plus $2,000 in cash, authorities have said.

Joel Wright, 23, who was arrested after he got off a plane in San Diego, California, following an undercover sting, had previously travelled to Tijuana, Mexico, in an unsuccessful attempt to adopt a child, the Department of Immigration and Customs Enforcement said.

According to the criminal complaint, Wright, who was expelled from Pontifical College Josephinum in Columbus, Ohio, tried in 2014 to adopt a child in Mexico and more recently spelled out in explicit online messages what he hoped to do with an infant and a four-year-old girl.

Asked in an email if he had previously had sex with infants, Wright allegedly responded: "I have not gone all the way before but I have made it very close in the past so I do have experance (sic)."

Wright is charged with travel with intent to engage in illicit sexual conduct and aggravated sexual abuse of a child. He will appear in court on Monday.

His mother, Teresa Wright Poquette, who lives in Vermont where Wright grew up, said she did not believe the allegations.

The Rev John Allen, vice president for advancement at Pontifical College Josephinum, said Wright began attending the school last autumn and had undergone a battery of psychological tests, interviews and a background check before being accepted.

Wright lost his status at the seminary when he left the campus without authorisation.

"We're shocked and saddened by the heinous nature of the allegations," Mr Allen said.

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