ActionScript 2.0 :: Flash 8 Referring To Code Created Objects

I'm creating mc's during runtime like this:
for (var i:Number = 1; i <= pictures; i++) {
this.attachMovie("box_mc", "box" + i+ "_mc", this.getNextHighestDepth())
}

So, if pictures is 5, then we end up with 5 boxes, box1_mc, box2_mc, box3_mc and so on. But I can't work out an easy way to refer to them later on. For example, say someone presses a button and I want to move all of them - how can I set their _x value in a loop? For example, if I wanted to move all of them 100 pixels, then this wouldn't work:
for (var i:Number = 1; i <= pictures; i++) {
"box"+i+"_mc"._x = "box"+i+"_mc"._x - 100;
}