Cannot remove old log files

[Paulino Calderon]

Hello.
I am currently moving to a new log administration policy, the problem
is that there are some old logs that I would like to delete but I
can't, the error is the typical "Operation not permitted" although I
am trying to do this as root, the file's permissions are ok and there
are no special file flag activated or anything, any ideas?

suckea# uname -a
FreeBSD suckea.com 5.3-SECURITY FreeBSD 5.3-SECURITY #0: Wed Jul 20
06:22:23 UTC 2005    
root at builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  i386

mybox# ls -lo
[snip]
-rwx------  1 root  wheel  -      78 Dec  6 15:00 auth.log.zzpKVF1
-rwx------  1 root  wheel  -      78 Jan  3 05:00 auth.log.zzr4dZD
-rwx------  1 root  wheel  -      78 Jan  4 17:00 auth.log.zzyP5R0

mybox# rm -rf auth.log.*
[snip]
rm: auth.log.zzr4dZD: Operation not permitted
rm: auth.log.zzyP5R0: Operation not permitted

mybox# uname -a
FreeBSD suckea.com 5.3-SECURITY FreeBSD 5.3-SECURITY #0: Wed Jul 20
06:22:23 UTC 2005    
root at builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  i386

Thanks for your time.