The notes in that intro are E, G, Bb, and A. There aren't enough notes here to form a standard scale or mode, but you could notice that E-G-Bb form a diminished triad and postulate an E Locrian mode. Given that F# makes an appearance later I might be inclined to consider E Locrian 2 (also called E Aeolian b5), which is the 6th mode of the G Melodic Minor scale. This might be a useful scale to use to improvise with in some parts of the song.
$A 7 $A 9 $A 10 $D 7 $D 8 $D 10 $G 7 $G 9
E Locrian 2 (E Aeolian b5)
Riffs are not always best thought of as derived from scales, though. The song as a whole is not in the Locrian 2 mode, but in the key of E minor. That Bb in the intro riff exists along side B as the song gets going, which is to say that both the b5 and the 5 of E minor are being used. It might be better to think of this riff in terms of chord tones and passing tones: The E and G are the root and 3rd of an Em chord, and the Bb is just a passing tone on the way to A. If you think of this A as the root of an A chord (or an A5) then this riff just follows a I-IV progression with a Bb passing tone.