No-one else was hurt in the crash
A handwritten note left in Charles Bishop's pocket "expressed sympathy" with the events of 11 September, however it indicated that the schoolboy had acted alone, said Tampa police chief Bennie Holder.
"From his actions we can assume he was a very troubled young man."
'Loner'
Bishop, a student at East Lake High School, Tampa, has been repeatedly described as a loner by his teachers and others who knew him.
"He never talked to anybody," said neighbour David Ontiveros.
However classmate Ross Stewart said that Bishop was a "teacher's pet" who got consistently good marks at school.
"He seemed to like his classes...he was never really down about anything (and) he smiled a lot," he said.
And his former algebra teacher, Rayette Bouldrick, said she was stunned by the news of his death.
"I'm floored...he always had a smile (and) he was always pleasant and respectful."
Police and FBI agents interviewed Bishop's relatives on Sunday, and are currently sifting through documents and information stored on the teenager's home computer, hoping to shed further light on his motives.
Police said that he had no criminal record or history of psychological or drugs problems and had not attempted suicide before.
Previous tragedies
America has been troubled by a spate of shocking teenage crimes in the past few years.
In April last year Californian teenager, 15 year-old Charles Andrew Williams, was accused of killing two classmates and wounding 13 others at his high school.
And in November last year three Massachusetts teenagers were charged with plotting a mass school shooting, which was foiled by police.
Most notorious is the 1999 Columbine massacre, when 12 students and a teacher were killed by two students armed with guns at the Colorado high school.
Dylan Klebold, 17, and Eric Harris, 18, then turned the guns on themselves.