Threefolds in P^5 with one apparent quadruple point

In this article we classify all the smooth threefolds in P^5 with an apparent quadruple point provided that the family of its 4-secant lines is an irreducible (first order) congruence. This is sufficient to conclude the classification of all the smooth codimension two varieties in P^n with one apparent (n - 1)-point and with irreducible family of (n - 1)-secant lines.