The results of the tests were announced in Brasilia by Human Rights Minister Ideli Salvatti.
She described the investigation as a victory for democracy.
"A dictatorship can order an assassination for political reasons, but only a democracy can investigate the reason for a political assassination," said Ms Salvatti.
Despite the announcement, Brazil's National Truth Commission, which started work in 2012, said it would carry on
investigating the circumstances around Mr Goulart's death.