DENVER -- Two F-16 jet aircraft from the 120th Fighter Squadron of the Colorado Air National Guard broke the sound barrier over the metro area Wednesday afternoon, creating a sonic boom that prompted dozens of worried phone calls to 911 operators from area residents.

Buckley Air Force Base said that the two jets were flying an "irregular air patrol" for North American Aerospace Defense Command in support of Operation Noble Eagle when they both exceeded the speed of sound, causing the sonic boom.

It was loudest over Westminster and Broomfield, according to people contacting 7NEWS.

"I felt it and I heard a boom and the whole house shook and the ground shook," said Velma Helker, who was in her Thornton home when the sound exploded between 3:18 p.m. and 3:23 p.m.

Colorado Guard spokeswoman Capt. Holly Peterson said it is not normal procedure for Air National Guard jets to break the sound barrier over a populated area. She said the pilots have given a report to operations personnel and their commander.

Colorado Air National Guard Brig. Gen. Mike Edwards said that the pilots could face action ranging from additional training to punishment.

"This was well into the mission. It just happened to be a mistake. This aircraft is a supersonic aircraft and if you're not careful you can exceed the speed of sound," said Edwards.

There are areas in the United States where jets are allowed to break the sound barrier, but Colorado does not have supersonic airspace.