Zombie Jails - why don't they disappear?

I've got zombie jails on a freebsd 6.x box (currently 6.2).
There are no processes running under any of them, but they still appear
on the jail list:

server# jls
   JID  IP Address      Hostname                      Path
     2  xxx.xxx.1.234    host1.domain		/data/jails/host1
     1  xxx.xxx.1.235    host2.domain		/data/jails/host2

I'm flumoxed as to why they're still there.  There aren't any processes
running in them:

server# ps -auxww | grep J

Any ideas why they are still hanging around in the jail list?

Joe