trying to find out which person made changes on a wordpress site

One of my clients has a wordpress site and has three people that are authorized to make changes to it. Only one specific person on the team normally makes the changes but some additional changes have been made to the site recently. He wants to know how he can track down who made the changes. Are there any log files that can be pulled to see who logged into the admin area within the past 30 days?

Corn