Where are the related files?

cron looks for files containing schedules at various places.

/etc/cron.d: this is a directory that contains cron jobs set up by other programs. If you have a software RAID system, you will find mdadm in this folder. This file specifies the frequency and the command to check the integrity of a RAID system.
/etc/cron.daily: this contains commands to run “daily” at midnight. Each file is a script (or binary command) that is run at midnight.
/etc/cron.hourly: this contains commands to run every hour.
/etc/cron.monthly: this contains commands to run every month.
/etc/cron.weekly: this contains commands to run every week.
/etc/crontab: this is the crontab file of the root account.
/var/spool/cron/crontabs: this contains user-specific cron job files. crontab -e handles files that will be placed here (except for the root account).

at looks for files here:

/var/spool/cron/atjobs: files that specify jobs. These files are created by at, and you should not hand edit these files without a very good reason.