First: I was actually using a prior WP version when the hijacking was done. I upgraded after fixing it. Not 100% sure what the prior version was. Probably 3.5.2.
My site was hacked by somebody who replaced or altered the index.php file in the twentyeleven theme and the index.php and footer.php files in the twentytwelve theme. They added a footer that had an image of a scorpion, played some music, and had a message saying that the site had been "hacked by equal". My "posts" page linked to their site, not to my posts.
I fixed the problem (I hope) by replacing the infected PHP files from backup.
Another weird thing happened. My admin username, which was NOT "admin" was CHANGED to "admin". I'm not sure whether this was done during the hack, or when I upgraded WP. Probably by the hack.
Appreciate any suggestions as to how to secure my site against such an attach in the future.