Well that is up for discussion. IIRC, all 19 homicide hijackers entered the US legally, but some overstayed their visas(thus making them illegal), but they weren't also truly and collqually illegal, since they entered the country to learn how to fly planes and kill innocent Americans and had money, while the standard American English colloquialism for illegal, is a Mexican or Hispanic, who sneaks over the border and just wants to get a job at such repectable professions such as vegetable picking or construction.
But sometimes idealougues try to blur the lines.