The teenager who flew a light aircraft into a Florida office building in "support of Osama bin Laden" had not taken the acne drug on which some blamed his behaviour.
Officials said that a port-mortem examination showed no trace of Accutane in Charles Bishop's system. It also found no traces of alcohol or drugs.
The 15-year-old was the only victim when he flew a single-engine Cessna into the 28th floor of the 42-storey Bank of America building in Tampa earlier this month.
The incident was initially thought to have been an accident, but police later revealed that a suicide note had been found in which the teenager expressed sympathy for Mr bin Laden and support for the attacks of 11 September. Police also found a prescription for Accutane in his home. The drug's label warns it may cause depression and psychosis, and in rare cases it may cause thoughts of suicide.
A spokesman for the Hillsborough County medical examiner's department said: "Everything was negative. We tested him for all the drugs of abuse: cocaine, opiates, amphetamines. We tested him for alcohol, he was negative for that. He was tested for Accutane.''
Investigators have not determined whether Bishop ever took Accutane. Traces remain in the body for at least two days.