July 26, 2013 -- Boeing is investigating how a piece of a military aircraft fell off and crashed in a Texas backyard, authorities told ABC News.

Nate Martinez and his girlfriend Melissa Gutierrez were sitting in their San Antonio home Thursday morning when they heard a loud noise outside, they told ABC affiliate station KSAT.

"I looked outside," said Martinez. "I kind of looked all the way around and I was surprised to see an aircraft part there. It was a little surprising."

The grey, cone-shaped aircraft part "was obviously part of a military aircraft that had somehow fallen onto their back patio area," a spokesperson for the San Antonio Police Department told ABC News.

"This is unsusual, very unusual," Peter Field, an aviation consultant who specializes in military aircraft accidents, told ABC News.

Field identified the piece as an aft ray dome, a rear piece of a Boeing C-17 cargo plane, which is currently in service in the United States Air Force.

"I'm amazed that this happened. It is not common for the military to lose any part of an airplane like this."

Field believes the aft ray dome fell while the aircraft was in-flight.

"I would bet you that the crew didn't even know that it had come off because this part of the plane is all the way in the back. They would not have felt a thing," he said.

According to San Antonio police nobody was hurt by the fallen aircraft part and there was no major property damage.

Still, Martinez and Gutierrez are concerned for their children's safety.

"It's very scary...It could have [hurt our children]," Gutierrez said. "That's what we are worried about. We're out here a lot barbecuing and it landed on our barbecue pit."

A spokesperson for Joint Base San Antonio-Randolph, an Air Force Base near Martinez's home, could not comment on the incident and directed ABC News to Peter Pedraza, a spokesman for the Boeing Company.

"Boeing is investigating the cause of an in-flight incident...Our highest priority is the safety and well being of those in the community and the passengers and crewmembers who fly aboard Boeing airplanes. We will continue to share information as the investigation progresses," Pedraza said.