Memory leak

I've noticed the last two days that the minecraft server uses a lot of memory. I tested this by fully rebooting my server. After 5 hours of uptime with 10 players online, I encountered serious issues:
http://i4.minus.com/iyHGeTgrmQRQ3.png
after stopping the minecraft server, this happens:
http://i6.minus.com/ibjEuNlb8u9AG6.png

Note that when the first screenshot was taken (with more than 80% mem usage) the minecraft server was totally empty, 0 players online.

I don't know if this is a known issue, if it's a java problem or minecraft related.