I don't know; I'm not sure I'd say they "never bothered to look them up." I've found the Flight755 page at A Candy Rose to be very helpful in learning about people--strangers, acquaintances, and friends to the Rs--who could fit into these very scenarios and were "looked into" by LE. (I'm not trying to say that the list proves that LE did all they could or anything. It's just an interesting read for reference and for brainstorming other possibilities.)
http://www.acandyrose.com/s-Flight755-15thStreet.htm
Henry Lee specifically challenged the idea that other avenues weren't pursued, whatever that does for you.