Flash MX 2004.
textAttrs.alignment
Property; a string that specifies paragraph justification. Acceptable values are "left", "center", "right", and "justify".
The following example sets the paragraphs that contain characters between index 0 up to, but not including, index 3 to justify. This can affect characters outside the specified range if they are in the same paragraph.
fl.getDocumentDOM().setTextSelection(0, 3);
fl.getDocumentDOM().setElementTextAttr("alignment", "justify");