But here is a "good" UFO report with an "unknown" conclusion:

On July 24, 1952, two Air Force colonels, flying a B-25, took off from Hamilton Air Force Base, near San Francisco, for Colorado Springs, Colorado. The day was clear, not a cloud in the sky.

The colonels had crossed the Sierra Nevada between Sacramento and Reno and were flying east at 11,000 feet on "Green 3," the aerial highway to Salt Lake City. At 3:40P.M. they were over the Carson Sink area of Nevada, when one of the colonels noticed three objects ahead of them and a little to their right. The objects looked like three F-86's flying a tight V formation. If they were F-86's they should have been lower, according to civil air regulations, but on a clear day some pilots don't watch their altitude too closely.

In a matter of seconds the three aircraft were close enough to the B-25 to be clearly seen. They were not F-86's. They were three bright silver, delta wing craft with no tails and no pilot's canopies. The only thing that broke the sharply defined, clean upper surface of the triangular wing was a definite ridge that ran from the nose to the tail.

In another second the three deltas made a slight left bank and shot by the B-25 at terrific speed. The colonels estimated that the speed was at least three times that of an F-86. They got a good look at the three deltas as the unusual craft passed within 400 to 800 yards of the B-25.

When they landed at Colorado Springs, the two colonels called the intelligence people at Air Defense Command Headquarters to make a UFO report. The suggestion was offered that they might have seen three F-86's. The colonels promptly replied that if the objects had been F-86's they would have easily been recognized as such. The colonels knew what F-86's looked like.

Air Defense Command relayed the report to Project Blue Book. An investigation was started at once.

Flight Service, which clears all military aircraft flights, was contacted and asked about the location of aircraft near the Carson Sink area at 3:40P.M. They had no record of the presence of aircraft in that area.

Since the colonels had mentioned delta wing aircraft, and both the Air Force and the Navy had a few of this type, we double-checked. The Navy's deltas were all on the east coast, at least all of the silver ones were. A few deltas painted the traditional navy blue were on the west coast, but not near Carson Sink. The Air Force's one delta was temporarily grounded.

Since balloons once in a while can appear to have an odd shape, all balloon flights were checked for both standard weather balloons and the big 100-foot-diameter research balloons. Nothing was found.

A quick check on the two colonels revealed that both of them were command pilots and that each had several thousand hours of flying time. They were stationed at the Pentagon. Their highly classified assignments were such that they would be in a position to recognize anything that the United States knows to be flying anywhere in the world.

Both men had friends who had "seen flying saucers" at some time, but both had openly voiced their skepticism. Now, from what the colonels said when they were interviewed after landing at Colorado Springs, they had changed their opinions.

Nobody knows what the two colonels saw over Carson Sink. However, it is always possible to speculate. Maybe they just thought they were close enough to the three objects to see them plainly. The objects might have been three F-86's: maybe Flight Service lost the records. It could be that the three F-86's had taken off to fly in the local area of their base but had decided to do some illegal sight-seeing. Flight Service would have no record of a flight like this. Maybe both of the colonels had hallucinations.

There is a certain mathematical probability that any one of the above speculative answers is correct -- correct for this one case. If you try this type of speculation on hundreds of sightings with "unknown" answers, the probability that the speculative answers are correct rapidly approaches zero.

Maybe the colonels actually did see what they thought they did, a type of craft completely foreign to them.

Another good UFO report provides an incident in which there is hardly room for any speculation of this type. The conclusion is more simply, "Unknown," period.

This type of UFO report, if it was received by Project Blue Book, was stamped "Insufficient Data for Evaluation" and dropped into the dead file, where it became a mere statistic.

Next to the "Insufficient Data" file was a file marked "C.P." This meant crackpot. Into this file went all reports from people who had talked with flying saucer crews, who had inspected flying saucers that had landed in the United States, who had ridden in flying saucers, or who were members of flying saucer crews. By Project Blue Book standards, these were not "good" UFO reports either.

But here is a "good" UFO report with an "unknown" conclusion: