Omar Mateen was freed three times by the FBI because he was not considered a threat.
It left him able to plan his gun rampage which left 50 people dead and 53 injured at a gay nightclub.
colleagues said he had been making inflammatory remarks, alleging terrorist ties.
FBI agents interviewed him twice and kept him under surveillance.
But they were unable to verify any ties and inquiries were halted.
In 2014, he was investigated again because of a link to Florida-born Moner Abu-Salha, 22, who became the first American to carry out a suicide bombing in Syria.
An FBI spokesman admitted that Mateen was interviewed three times in total but the investigation was closed on each occasion because he was not considered a threat.
He added: “Contact (with Abu Salha) was minimal and did not constitute a threat.”