From: http://www.latimes.com/nation/la-na-thomas-eric-duncan-vigil-20141008-story.html
“Karsiah Duncan, who also goes by Eric, had invited his father to Texas this summer to attend his high school graduation. The visa didn’t come through until August, relatives said. He had not seen his father since he was 3 years old, when he and his mother immigrated to the U.S. The elder Duncan stayed behind in Africa.”
I read an earlier report that he came on a “Fiance Visa”. ( http://www.uscis.gov/family/family-us-citizens/fiancee-visa/fiancee-visas ) Those take a few months to process. In any case, it appears that the Visa request process was initiated well before he was exposed. The critical item of interest seems to be: When did he buy the tickets?” Most people will buy those far enough ahead of time to get a lower price.