Detroit (UPI) -The U.S. Air Force closed its books today on the case of two "visits” by unidentified flying objects. It’s verdict: swamp gas.

But some of the people who witnessed what they thought was a landing by a glowing, extra-terrestrial space craft were not completely convinced that the experts were right.

And even while the two most spectacular Michigan sightings now wore the tag of an “official explanation,” UFO sightings continued to pop up across the country, in rural areas an cities alike.

Dr. J. Allen Hynek. the top Air Force adviser on UFO sightings, climaxed three days of on-the-scene investigation by telling a press conference it appeared “very likely” that swamp gas was responsible for mass reports of strange glowing objects this week at Hillsdale and Dexter in southern Michigan.

Nynek, an astrophysicist from Northwestern University, said rotting vegetation can release a gas, especially at springtime, which can glow like fire and even make “popping” noises. He said both instances he investigated took place in marsh land.

“It appears very likely that the combinations of the conditions of this particular winter - an unusually mild one in this area — and particular weather conditions that night — were such as to have to produce this unusual and puzzling display,” he said.

Frank Manner and his son had described the object that hovered in a marsh at their farm near Dexter as football-shaped with a pitted surface, "like coral rock.” But Hynek, told reporters Manner and his son were 500 yards away from the “visitor” and couldn’t have distinguished such detail. He said distance also was a factor in the Hillsdale case, where 87 coeds, their house mother and a civil defense director watched an object in a nearby swamp for several hours.

“A dismal swamp is a most unlikly place for a visit from outer space.” Hynek said.