I remember hearing rumors back in the day that the Columbine shooters made maps of the hallways of their school for Doom I and II.
Is this true?

That was indeed a rumor that went around, but no such maps are known to exist (more info).

They did make maps but they bore no resemblance to their school, at least the ones that were on the internet. Either they did make such a map and simply never put it online (for obvious reasons) or paranoid idiots who dislike violent games pulled that rumour out of their arse to discredit Doom and failed.

http://doom.wikia.com/wiki/Columbine_High_School_Massacre

Their released WADs such as UAC Labs did however have rather disturbing text in them that betrayed their poor mental health (nothing to do with Doom or other games) such as threatening to kill anyone who broke copyright on their WADs. Because of this, had they not gone crazy and killed themselves afterward I bet they probably would have worked as lawyers at EA, Hasbro, Nintendo or Konami.

Maybe they would have been good friends with the Romaine brothers.

I know at least one map made by one of the Columbine killers is available for download at Doomworld. Never tried it myself, though.

tl;dr: The shooters were doomers and made a map named UAC Labs or something like that. The school was not a map, and in fact there is nothing out of the ordinary outside of some now disturbing text and the fact that it's above average.

However, maps of schools were all the rage back in the day because it was easy as heck to make a WAD out of your school. You can find plenty of those on /idgames if you look, but most of them are innocent.

That would be UAC Labs; as I understand it, they leave it up for historical purposes. I wouldn't bother with it, as it's rather unremarkable. The map itself is nothing special, and the slightly gorier sprites are tame, even by pre-Brutal Doom standards. It's only unsettling due to the context surrounding its existence.

Eric and Dylan did not ever make a map like that, but really strange Doom modders DID make a Columbine mod for Doom though.

Not linking it here for obvious reasons, using the search engine of your choice will help you if you are morbidly curious.