Sacramental confession and U.S. law have coexisted for 225 years now, and I have never heard of a case where a murderer confessed to a priest face to face but did not repent (which would raise the question: then why did he confess? --- understanding that the priest could refuse absolution unless he surrendered himself to the police.)
There have been novels along these lines, but no actual cases that I know of.
Which reminds me:
Ted Kennedy’s receiving a Catholic funeral was defended on the grounds that there was a priest oozing around the compound in the days before he died. I.e., according to canonists, the presence even of such a Mafia priest constitutes some sign of repentance.
If I had been his confessor, I would have made a public repudiation of his public sins—support for abortion and other social evils—a condition of absolution.
Whether there should have been the canonization that Cardinal O’Malley allowed—and clearly reveled in—is a whole ‘nother issue.
You make a good point regarding “how would the priest know who the person was”, but since Vatican II penitents can request absolution face-to-face or not. So, a priest can know who the penitent was.
Up to this point, I hadn’t thought of this specific example and only adds to my beliefs that changing to face-to-face confessions was a mistake.