Crown witness Peter La Serpe, who has to act as a witness in Willem Holleeder's appeal this year, has disappeared from the face of the earth.

According to the court, the Public Prosecution Service (OM) has not been able to contact him for months, as it turned out on Monday in the extra-secure court at Schiphol.

It is not clear what influence the possible inability to hear from La Serpe will have for the appeal.

The man's statements have been used by the court for evidence, but the court will have to rule again on the reliability.

In 2006, La Serpe concluded a deal with the judiciary in the large liquidation process Passage.

He admitted his own role in the shooting of Kees Houtman in 2005 and provided information about the murder of Thomas van der Bijl in 2006.

About Holleeder, La Serpe stated that he heard the words "Osdorp first" during a chance meeting.

Houtman lived in the Amsterdam district of Osdorp and Holleeder would therefore have given priority to the murder of the criminal.

One of the prosecutors said Monday it is "a mystery" where La Serpe is located and that it is "completely unknown" why contact has failed.

According to the Public Prosecution Service, there is no question of negligence.

"The possibility is that something happened to him without being aware of it."