What files are source in bash login shell?

[Peng Yu]

I make my ~/.bash_profile empty. I still see

$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games

I checked /etc/profile and /etc/bash.bashrc. But I don't PATH is set
in the two files. I'm wondering from which file PATH is set. BTW, my
system is ubuntu.