Charles Fox (music) and Norman Gimbel (lyrics) - they also did the theme to "Happy Days" (amongst others). Apparently the song was inspired by the feelings original artist Lori Lieberman had when she felt Don Maclean was singing to HER at a gig - it seems Gimbel expanded on a poem she wrote. The song became a hit for Roberta Flack, and then the Fugees did their famous version. I assume it was one of them who added "one time, two time".