Recently, he posted more than 100,000 pages of documents on the US Air Force`s internal UFO investigations to the internet. Here are the top five things to know from the open files of Project Blue Book.
1. Project Blue Book had a sizeable mission
The origins of the ambitious project can be traced to June 1947, UFO researcher Alejandro Rojas tells the BBC.
The editor of Open Minds magazine says a well-respected businessman and pilot, Kenneth Arnold, was flying over Washington state when he witnessed several unidentified flying objects.
Arnold later described the crafts as "skipping like saucers", which the media adopted and took to calling flying saucers.
This high-profile incident - along with several others, including a rumoured UFO landing in Roswell, New Mexico, the same year - led the Air Force to launch an investigative body.
The Air Force says this wreckage, recovered near Roswell, New Mexico, came from a radar target
Named Project Blue Book and headquartered at Wright-Patterson Air Force Base in Ohio, the programme was reportedly comprising only a handful of staff.
Nonetheless the group investigated 12,618 UFO sightings in a two-decade period.
2. Project Blue Book was created in a time of public unease
Formed in the years immediately following World War Two, Project Blue Book was intended to stop the spread of public unease about a growing number of reported UFO sightings, including over such landmarks as the White House and US Capitol.
"There was a lot of hysteria with the public, and that to the military and government at the time was a big threat in itself," Greenewald says. "It didn`t matter if UFOs were alien or not, they were causing a panic, so [the government] had to settle everybody`s nerves."
Though frequently met with derision today, UFO sightings are said to have been discussed at the top levels of government in the 1940s and 1950s.
"It was taken very seriously back then," Rojas says, with Central Intelligence Agency chiefs publicly claiming it was a real phenomenon and even then-Congressman Gerald Ford warning it needed to be investigated.
In 1966 a separate Air Force committee was set up to further delve into some of the cases within Project Blue Book. That group later released a report finding no evidence of UFO activity.
Project Blue Book was officially shuttered in 1969.
3. Many of the Project Blue Book cases appear open-and-shut
Though many credible sources, from Navy admirals to military and civilian pilots, reported seeing UFOs, most of the cases investigated by Project Blue Book were deemed caused by weather balloons, swamp gases, meteorological events or even temperature inversions.
In Seattle, Washington, in April 1956, a witness described seeing a "round, white object, one-half the size of the moon … [and] going round and round", according to documents.
Investigators later concluded it was a meteor and closed the case.
Amateur UFO investigators believe the truth is still out there
In January 1961 in Newark, New Jersey, a witness reported viewing a dark grey object "about the size of a jet with no wings".
That object was later deemed a jet aircraft flying in the area.
4. Some Project Blue Book cases aren`t so easily explained
According to Greenewald and Rojas, more than 700 Project Blue Book entries could not ultimately be explained by investigators. Many such cases cited insufficient data or evidence.
But even some of the closed cases raise more questions than answers for UFO researchers.
In one such example, a police officer in 1964 in Socorro, New Mexico, halted vehicular pursuit of a suspect after he saw a strange aircraft overhead.
The officer followed the craft - which he described as bearing a strange red insignia - and saw it land and two child-sized beings exit.
It later took off, leaving scorch marks and trace evidence on the ground.
"[Project] Blue Book labelled it unexplained; even after all these decades they still can`t explain it," Greenewald says.
5. There is still information to be uncovered about UFO activity
Though Greenewald has amassed a stockpile of government documents, he says there are still many he - and the public - has not yet accessed.
One request to the National Security Agency yielded hundreds of pages, but they were so redacted only a few words were readable on each page, he says.
Other US government entities - including the Central Intelligence Agency and Defense Intelligence Agency - also conducted UFO investigations that have not been publicly released, Greenewald notes.
"I think Project Bluebook … is simply the tip of the iceberg," he says, adding he will continue to request more information from the US government.
"There are secrets after conspiracies after scandals that continue to come out," Greenewald concludes. "There`s always something to go after."
