In Sir Arthur Conan Doyle’s classic tale “The Sign of Four,” detective Sherlock Holmes instructs Dr. Watson thusly: “Eliminate all other factors and whatever remains, however improbable, must be the truth.”

Eliminating all other factors is South Jeffco resident John Schuessler’s primary occupation, and he hopes to someday uncover the truth about the baffling aerial phenomenon commonly referred to as Unidentified Flying Objects – UFOs.

Speaking to a capacity crowd at Littleton’s Bemis Public Library recently, Schuessler recounted some of Colorado’s more dramatic UFO sightings and detailed how the organization he helped found 40 years ago is trying to bring the mysterious lights in the sky to earth.

Littleton resident and Arapahoe Community College business professor Jerry Thomas came to hear Schuessler for the least mysterious of reasons.

“It sounds like an interesting topic,” he said.

Not one to volunteer the information, when pressed Thomas recounted how he and a buddy, while stationed aboard a Navy vessel stationed in the South China Sea many years ago, saw “something vertical” hovering on the near horizon. As they watched, the object turned 90 degrees and shot out of sight.

“It took off like a rocket,” Thomas said. “At exactly the same time we both said ‘what the hell was that?’” So, you’d call that a UFO? “Well, it was unidentified to me,” he laughed, and took his seat.

According to Schuessler, reports of weird things flitting around above the Centennial State date back at least to 1926 when a stunt pilot claimed to encounter six “giant manhole covers” in the air above Colorado Springs.

“There were no ‘UFOs’ in 1926,” Schuessler said, “so he just described them as he saw them.”

More recently, numerous witnesses reported seeing a large circular object hovering over AT&T’s Denver laboratory in 1985. Lingering for several minutes in full daylight at low altitude before whizzing off, the great, shiny disc generated lots of consistent testimony but no easy answers.

In 1995, Schuessler said, two UFOs were sighted by numerous observers in Leadville and, just a couple of years ago, a tornado chaser perched on the hogback anticipating a rough late-spring afternoon was startled by a pair of “large bowling balls” floating past on a northeasterly course toward Denver International Airport.

Perhaps Colorado’s most spectacular UFO sighting occurred in 2004 when scores of picnickers at a park on Estes Street in Lakewood reported seeing an enormous disc appear in the clear sky overhead.

“A lot of them had video cameras” Schuessler said, “and lights on the object can be seen pulsing at 8.26 hertz, right about the limit of what the eye can detect.”

Although Schuessler didn’t bring any videotapes to the presentation, he brought plenty of pictures. Unfortunately, he only managed to display one intriguing – and very clear – image of three stationary discs parked in the sky over Denver International Airport before the light bulb in the library’s overhead-projector burned out. When the backup bulb was found to be inoperable, some in the audience suggested the situation exceeded the definition of coincidence.

Schuessler’s involvement in UFO research started in 1965, when he worked at the Johnson Space Center in Houston. In charge of life-support systems for NASA’s Gemini IV manned space mission, Schuessler became friends with the two astronauts detailed to the project, one of whom took several photographs of a strange object that shadowed the Gemini capsule as it orbited the earth. Back on earth, NASA technicians processed the film and dumbfounded the astronaut by returning pictures that were clearly not of the object he’d seen in space.

“It was obvious they weren’t the pictures he’d taken,” Schuessler said. “It got my attention.”

In 1969, together with a handful of other curious sorts, Schuessler launched the Mutual UFO Network, or MUFON, dedicated to investigating and documenting flying singularities or trace evidence thereof. From humble beginnings, MUFON has grown to more than 3,000 regular members from all walks of life – including lifetime-member Dan Aykroyd – and nearly 500 trained investigators and who study sightings from pole to pole. Recognized as a leading authority on UFOs, Schuessler has appeared on dozens of television programs exploring the subject.

Once upon a time Schuessler kept a storefront at Chatfield Avenue and Kipling Street in Littleton where Colorado MUFON volunteers Ginger and Jack Sawatzki once spent hours sorting and filing countless faxed reports before the increasing volume and international scope of UFO sightings prompted Schuessler to close the shop and shift reporting to the internet. Though Ginger’s never actually seen a UFO, she was one of many whose interest originated in Roswell, N.M., the alleged site of a space vehicle crash more than a half-century ago.

“I’m troubled that it’s still being kept secret after 60 years,” she said. “There’s more going on than they’ll admit.”

Though most UFO reports originate in America, MUFON receives accounts from nearly every country. China and Russia are particularly prone to irregular aviation, and California is the hands-down leader of UFO activity in North America with almost 30 reports in April, alone.  Colorado, by contrast, is near the middle of the pack with six reports last month. Each sighting is judged by two basic criteria.

“We look at credibility and strangeness,” Schuessler said. “Who sent the report and what did they see?”

Luckily, a large number of UFO reports are filed by police officers and airline pilots, trained observers who can generally be trusted to keep their head, provide important details and not exaggerate the facts. The second aspect, strangeness, is more subjective.

“A light in the sky isn’t strange,” Schussler said, “one that travels at high speed and makes a 90-degree course change is.”

Of the tens of thousands of UFO reports MUFON has fielded over the years, only about 6,000 – roughly 10 percent – have merited a closer look. Promising cases are assigned to highly skilled volunteers, many of them scientists working in diverse disciplines like engineering, astronomy and photo analysis who examine weather reports, flight logs and a host of other sources in order to eliminate more conventional phenomenon that could explain the sighting. In most cases, mysterious fliers are revealed as something perfectly prosaic like small aircraft, weather balloons or flying squirrels, but the small percentage that defies explanation is what keeps MUFON growing.

“People make mistakes, of course, but they can’t all be wrong.” Schuessler said. “The evidence for UFOs is overwhelming.”

Following the attack on the World Trade Center, Schuessler said, some of that evidence is getting harder to come by. Before 9/11, the Federal Aviation Administration routinely made pilot transcripts available to MUFON investigators, but no longer. Fortunately, detailed radar information is still obtainable through various channels and still goes a long way toward proving or disproving UFO sightings.

In Schuessler’s highly methodical operation, most UFOs classified as circular or disc-shaped, followed closely by orbs and cigar-shaped objects. Flying boomerangs are less common, and sightings of gigantic triangles with or without lights are rare but prized. According to MUFON statistics, 75 percent of UFOs appear in clear skies, and a surprising number appear during daylight hours. For reasons Schuessler can’t fathom, UFO activity peaks on Wednesdays.

The most valued observations are CEs – close encounters – viewed from a distance of less than 500 feet.

“It would be pretty hard to mistake something that close,” Schuessler said. Worldwide, 15 CEs were reported last month.

When questioned about the presumed link between UFOs and crop circles, Schuessler hedged. “I don’t study crop circles just because there are so many people already doing that,” he said.

Similarly, he wouldn’t comment directly on the subject of UFO abduction but granted that, given the remarkable things he’s seen in 37 years of study, the possibility for it certainly exists.

A common misconception is that UFO researchers presume a link between UFOs and extra-terrestrials, something Schuessler flatly denied.

“I don’t know who they are or where they come from,” he said. “But I’d love to know before I stop doing this job.”