stuck in doom1 level 3

[New2Doom]

In level 3 you need to proceed a building and at the end you find a blue keycard and all monsters come out, after having destroyed all of them I'm stuck and the only door I didn't open yet because I can't is the one were I need the red key for.
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help?

[Illdo]

http://www.gamefaqs.com/computer/doswin/game/524287.html