It was just before 11 a.m. Wednesday when Bellevue police Capt. Herb Evers heard the boom.

Evers was in the city's fleet maintenance facility, talking to a mechanic.

"The windows in the garage doors rattled," Evers said. "It was like you could feel the concussion."

He thought a plane had exploded and called the base police at Offutt Air Force Base. The person who answered the phone said, "Yeah, I know. We heard it, too."

They weren't alone.

People heard the boom from Cunningham Lake on Omaha's northern edge to Cass County, from La Vista to the west end of Council Bluffs.

Sarpy County emergency dispatchers fielded about 100 calls, while Douglas County dispatchers were just as busy. OPPD also received a number of calls.

Officials ruled out a number of things the boom wasn't: Fire. Explosion. Earthquake. Meteorite.

So what caused metro-area residents to momentarily question their safety around 11 a.m. Wednesday?

Most authorities are content with believing that the noise was the result of an airplane exceeding the speed of sound, an event known as a sonic boom.

Offutt has no planes that can make a sonic boom, but an F-16, which can fly at twice the speed of sound, was flying over the central United States Wednesday, said Lt. Col. Les Carroll of McEntire Joint National Guard Base in Eastover, S.C.

He's still investigating to see if the plane was responsible for the boom heard in the Omaha area.

Carroll said planes aren't allowed to fly fast enough to create a sonic boom over U.S. land, except over some remote areas. They do conduct training missions over the ocean, where speed isn't restricted the same way.

"It's rare," he said. "I'm sure if someone is responsible, they didn't intend to do it."

Carroll did not identify the plane's destination, but said it was scheduled for a paint job.

The lower the aircraft is traveling, the louder a sonic boom would sound, said Ken Plotkin, chief scientist at Wyle Laboratories in Virginia.

He said sonic booms aren't dangerous, but they can be startling to unsuspecting people and damaging to fragile objects, such as glass.

Reports Pour In About Big Boom 20 June 2007

A loud boom was heard before noon, and Sarpy County officials said they are checking on what may have caused it.

KETV NewsWatch 7 received several calls from people who said they heard or felt a boom in Douglas, Madison, Sarpy and Cass counties in Nebraska, and Council Bluffs, Iowa. Across the region in cubicles and neighborhoods, the boom was all people could talk about. Shirley Jefferson, in Irvington, said the boom shook her house. Treasure Baker, who lives at 1917 Military Ave., said her house shook and the dog went crazy.

People as far away as Tabor, Iowa, and Fremont, Neb., also reported the boom.

"Just like a clap of thunder -- loud thunder -- between two houses," one witness told KETV NewsWatch 7.

"I work around 60th (Street) and Ames Avenue, and I also thought somebody came down our alley and hit our building," one poster wrote in a forum.

Experts said the big boom was most likely sonic, though air traffic controllers from Eppley Airfield to Offutt Air Force Base said they saw nothing unusual in the skies and nothing that would make a sound as loud as people described. Offutt air traffic manager Don Hughes said there was an F-16 flying high overhead at about 18,000 feet. As it was making its descent to Sioux City, Iowa, it was clocked at about 530 knots, or just less than the speed of sound. Hughes said that under the right atmospheric conditions, that speed could cause a sonic boom.

As of 5 p.m. Wednesday, the speed of the planes had not been confirmed.

Flying faster than the speed of sound is illegal over land.

Experts said that sometimes a moisture cloud forms around airplanes as they break the sound barrier. In rare occurrences, a sonic-boom cloud can be dramatic.

"On a day like today, the air would be a little less dense, (decreasing the speed of sound,)" said Creighton University physics profess Dr. Michael Cherney. "A sonic boom is a kind of shockwave. What happens is, if the object creating the shockwave goes faster than the wave can go, all the waves created build up to give you a great big wave. That's the bang you hear as a sonic boom."

The Sarpy County sheriff said he had investigators look at a gas regulator that was found on its side on Sheridian Road and 36th Street between Bellevue and Papillion. Just before 3 p.m., the Metropolitan Utilities District said it looked at the regulator and didn't find any disturbance or leaks.