Large chunks of ice, one of them reportedly about 50 pounds, fell from the sky in this northeast Iowa city, smashing through a woman's roof and tearing through nearby trees.

Authorities were unsure of the ice's origin but have theorized the chunks either fell from an airplane or naturally accumulated high in the atmosphere _ both rare occurrences.

"It sounded like a bomb!" 78-year-old Jan Kenkel said. She said she was standing in her kitchen when an ice chunk crashed through her roof at about 5:30 a.m. Thursday. "I jumped about a foot!"

She traced the damage to her television room, where she found a messy pile of insulation, bits of ceiling, splintered wood and about 50 pounds of solid ice.

Karle and Mary Beth Wigginton, who live a block away, heard a loud "whoosh" coming through the trees. They discovered several large chunks of ice in front of their home and some smaller ones in the yard and in the street.

"I could see where branches were shredded, which told me it was definitely coming out of the sky," Karle Wigginton said.

He estimated the original chunk of ice was the size of a basketball. "It was pure white," he said. "The main parts I picked up were very smooth."

Elizabeth Cory, a spokeswoman for the Federal Aviation Administration, said investigators would contact Kenkel to try to determine the source of the ice.

"It is very uncommon for something like this to come from an aircraft," Cory said. "That is really unusual if it is pure white ice, especially at this time of year."

Occasionally, aircraft latrines discharge contents at altitude, resulting in chunks of descending ice. Airplanes also sometimes accumulate ice on their edges in certain atmospheric conditions, including high altitude and extreme moisture, said Robert Grierson, the Dubuque Regional Airport manager and a pilot.

The moisture involved in such a scenario could have come from the tops of strong thunderstorms. However, Dubuque had clear skies at the time the ice fell, said Andy Ervin, a meteorologist with the National Weather Service in Davenport. "There was nothing unusual going on," he said.

David Travis, a professor of geography and geology and an associate dean at the University of Wisconsin-Whitewater, has studied the phenomenon of large chunks of ice falling from a clear sky. He said it's possible the ice could have been a megacryometeor _ "similar to a hailstone, but without the thunderstorm."

Travis is part of a research team that has documented more than 50 possible megacryometeor cases during the past five years. Some involve ice chunks the size of microwave ovens.

"It is hard to keep something like that suspended in air without a thunderstorm," Travis said.

Most megacryometeor sightings have occurred in coastal areas, where atmospheric turbulence helps keep ice suspended long enough to grow into large chunks.

Travis' research team speculates the phenomenon could be linked to global warming, suggesting that climate change might make the tropopause portion of the atmosphere colder, moister and more turbulent.

"But those don't typically happen in the summer time," Travis said. "It seems like they are mostly associated with the passage of passing cold fronts."