Finding delete or update records

Question

I have had three incidents in recent months where records in a table that have either been deleted or values updated to zero across a whole table. We have a team of four people who have the permission and who are responsible for updating the database who could have done this. Disappointingly no one has admitted to carrying out the changes.

Going forward I would like to be able to have a record of these transactions. I was wondering what do other people use to track these changes? Do they use software that tracks changes or do you create stored procedures or trace files? If anyone has this set up at their facility I would like to know what they use. The trace files do have the information I am after such as login name machine number and the sql statement so it will give me the information if I set them up in advance.

I have copies of the database and the transaction logs when these changes took place. Is there anything I can do with these old files to help track down the culprit? Thanks in advance to anyone who replies. We are using SQL server 2005.