Attended II Vatican Council, 1962-1965; (This alone is sufficient to prove his heresy, by participating and presiding over Vatican II without protesting the many heresies there. A heretic cannot be Pope and a Pope cannot become a public heretic.)
member of its board of presidency, 1963-1965.
Participated in the conclave of 1963. (By participating in the conclave, Siri admits its validity.)
A single eye witnessTran Van Khoat, ordained in Vietnam in 1968 (and therefore una cumAntipope Paul VI, which makes him a schismatic per se.) claims that Siri claimed the Papacy. Canon Law requires two witnesses, but we have only one, which is sufficient to set aside Khoat's testimony. Further Khoat's position and testimony varied over time, which causes one to question his credibility. Therefore Tran Van Khoat's testimony must be set aside.
Siri died on May 2, 1989, having never publicly claimed the papacy. Since who is Pope is a Dogmatic Fact, necessary for people to know, the lack of this claim is sufficient to set aside his claim. Siri never publicly appointed Cardinals, nor publicly changed Canon Law, therefore his secret Cardinals died with him, as provided by the 1917 Code of Canon Law, which would be in force, if Siri had been elected in 1963 (early reports) or 1958 (later reports).