His investigation further leads him to an unsolved mystery 20 years ago.

The origin of the fire remains an unsolved mystery.

With no further investigation, the case had, since then, gone cold over the years, eventually acquiring the status of an unsolved mystery.

It examines the background leading to the catastrophe and probes some of the unsolved mysteries that still surround the sinking of the cruise liner.

If he were to come forward, he would end one of the great unsolved mysteries of the 20th century.