Corrina Vargas, 26, was cooking breakfast in her mother's neighborhood home Monday morning when she heard an explosion.

Her son, Carlos Mendez, 10, was sitting on a bed in the living room watching cartoons when something came through the roof. A small piece of the debris hit him in the back of the head.

They ran outside, and Vargas' mother, Mary Montano, gathered the rest of the adults and children in the house and got them out.

No one was hurt by what turned out to be a bowling ball-sized chunk of ice that crashed through the roof. The hole in the roof appeared to be two to three feet wide.

Police spokesman Sgt. Craig Gundlach said investigators were not sure where the ice came from, but it may have dropped from a commercial airliner.

"We are in contact with the (Federal Aviation Administration), and they are not aware of where it could have come from," Gundlach said. "There is no indication it was anything but an accident."

FAA spokesman Ian Gregor said investigators were en route to Modesto to examine the ice and try to determine where it came from.

It may be "blue ice" that leaked from an aircraft, Gregor said. Airplane bathrooms can sometimes leak, and the effluent, containing human waste, forms a chunk of ice outside the plane, Gregor said.

It is frequently blue because of the blue liquid used to flush toilets in planes. The air at the altitude airliners fly can be 50 degrees below zero, Gregor said.

When the airplane descends, the ice thaws and sometimes falls off the plane, he said.

Falling blue ice from airplanes is reported around the country from time to time. An incident was reported a few months ago in Ontario, California east of Los Angeles, Gregor said.

How often blue ice falls from airplanes is difficult to track, because if the ice doesn't hit a home or damage anything, no one would know, Gregor said.

"It's not common, but it's not unheard of," he said.

Some commercial planes are prone to bathroom leakage, according to a 2004 FAA directive. Several models of the Boeing 737 are required to undergo repetitive leak checks and bathroom maintenance.

Modesto is under the flight path of airliners coming into San Francisco International Airport, Gregor said. The planes are at a little more than 20,000 feet when they pass over, he said.

Gundlach said the ice didn't appear to have come from a low-flying plane arriving or leaving Modesto Airport, which is across the street from the house.

If it were from an airplane flying at 20,000 feet, a chunk of ice that size could reach a speed of 120 to 200 mph, according to Joseph Alward, a physics professor at University of the Pacific. Alward estimated that the ice chunk may have been traveling at 160 mph.

The ice chunk hit the house with explosive force, according to Vargas.

"It was just a big old boom, and all that debris," she said. "I was just stunned. I thought it was a fire from the water heater."

There were four adults and five children inside when the ice hit it, Gundlach said.

The ice punched a sizable hole in the roof, broke through a roof truss and continued through the living room ceiling, he said. Had it not shattered as it penetrated the home, it would have landed about a foot from where Carlos Mendez was sitting, Gundlach said.

"With the velocity that chunk of ice had, we are very lucky no one was severely injured or killed," he said.

Firefighters said the fragments were brown and white, and looked like dirty ice. They found two brick-shaped chunks and lots of smaller shards.

Gundlach said evidence would be turned over to the FAA.

There is a possibility that the ice chunk wasn't from a plane.

Several incidents have been reported around the world of large chunks of ice falling to the ground that are apparently unrelated to airliners.

Jeszs Martmnez-Frias, a Spanish geologist, thinks the ice may form in the atmosphere, similar to the way hailstones are created.

He coined the term "megacryometeors" for the ice chunks. His hypothesis is controversial, however. Other scientists question whether ice chunks that large could be formed in that way.

FAA investigators may know more in the next few days, Gregor said.