can't unlink a file ??? (hacked...)

[skanx]

following a security problem (been hacked...), I have some files who belong to the "reboot" group. The problem is that these are some files in the /var/log : messages, xferlog, etc...

I can't change their group :
#chgrp root /var/log/messages returns :
chgrp: you are not a member of group `root': Operation not permitted
although i'm root and member of root! (i tried newgrp root, no success...)

and I can't delete them :
#rm -f /var/log/messages returns :
rm: cannot unlink `/var/log/messages': Operation not permitted

what can I do to change the group ???
any help greatly appreciated

 

[skanx]

that was the lsattr and chattr commands (attributes) I had to use...