Luis Elizondo made the remarks after having ran a $22million (£15.9million) US Pentagon programme to study unidentified flying objects from 2008 to 2012.
Money was granted for the programme, known as the “Advanced Aerospace Threat Identification Program” (AATIP) at the behest of former US Senator Harry Reid.
Mr Elizondo said: "My personal belief is that there is very compelling evidence that we may not be alone.”
Whilst he admitted that he could not speak on behalf of the US Government, he has before strongly suggested that there was evidence that stopped him from ruling out the possibility that alien aircraft visited our planet.
Speaking of the objects that were researched, Mr Elizondo said: "These aircraft -- we'll call them aircraft -- are displaying characteristics that are not currently within the US inventory nor in any foreign inventory that we are aware of.”
The former Pentagon official also claimed that those working on the AATIP managed to identify "anomalous" aircraft that were "seemingly defying the laws of aerodynamics”.
He added: "Things that don't have any obvious flight services, any obvious forms of propulsion, and manouvering in ways that include extreme manouverability beyond, I would submit, the healthy G-forces of a human or anything biological,"