If indicted, he will have to stand trial in the Vatican and faces up to 12 years in jail.
After a month-long investigation, detectives have arrested a St. Petersburg man and charged him with multiple counts of possession of child pornography, the St. Petersburg Police Department said.
The Vatican said on Saturday a monsignor who was a former advisor at its United States embassy in Washington has been arrested on suspicion of watching child sex abuse.
The following month, police in Canada issued a arrest warrant for Capella.
A Vatican statement identified the accused as Msgr.
The case is the latest blow to the Catholic Church as it struggles to overcome repeated sex abuse scandals among its clergy.
The Italian monsignor was recalled to the Vatican from Washington after the US State Department notified the Holy See on August 21 of his possible violation of laws relating to child pornography images.
The statement said that Capella is being held under the authority of a set of 2013 Vatican criminal norms.
The United States made "an official request" for the man's diplomatic immunity to be lifted but the Vatican refused, said a USA official on condition of anonymity.
He is being held under paragraph 3 of article 10 of Law 8, which applies the penalty of at least one to five years imprisonment and a fine of 2,500 to 50,000 euro for the distribution, dissemination, transmission, or sale of child pornography, or its possession for these purposes.
The arrested monsignor was deployed at the embassy, in Washington since mid-2016.
Archbishop Wesolowski was recalled to Rome by the Vatican, arrested and stripped of his duties after a report by the Dominican news media led to an investigation by Dominican magistrates.
He died in hospital in 2015 at the age of 67 before his trial before a Vatican court could begin.