What to Know The White House was locked down and fighter jets were mobilized after a reported airspace violation in Washington, D.C., officials said.

"Upon further investigation, we found there was no aircraft," a NORAD spokesman said in an update.

The airspace in and around D.C. is more restricted than in any other part of the country.

The White House was locked down and fighter jets were mobilized to respond to a report of an airspace violation in Washington, D.C., Tuesday morning — but it turned out to be a false alarm, a federal official told News4.

The North American Aerospace Defense Command (NORAD) identified an object on radar that appeared to be an aircraft, spokesman Maj. Andrew Hennessy said in an update Tuesday afternoon.

A U.S. Coast Guard helicopter was sent to investigate the apparent aircraft inside protected airspace east of the U.S. Capitol building.

"Upon further investigation, we found there was no aircraft," Hennessy said.

Objects spotted in the air on radar are sometimes weather balloons or flocks of birds, the NORAD spokesman said, noting that it's still unknown what caused this particular scare.

The investigation is ongoing.

NORAD said on Twitter at about 9:30 a.m., "Senior interagency officials are monitoring the situation on a national event conference call. Our jet fighters are on site and responding. Plane is not considered hostile at this time.

The White House was locked down shortly before 9 a.m. That lockdown was lifted about 20 minutes later, NBC News reported. The North Lawn of the White House was cleared. The U.S. Secret Service said personnel at the White House were told to remain in place.

"The White House was locked down this morning due to a potential violation of the restricted airspace in the National Capital Region," a Secret Service spokesman said. "The lockdown has been lifted at this time."

The U.S. Capitol and its office buildings were evacuated as a precaution for about half an hour. Capitol Police sent a notification of a potential threat shortly after 8:30 a.m., and the evacuations were ordered. People were allowed back in after about 45 minutes.

Neither the Federal Aviation Administration (FAA) nor the Department of Homeland Security answered questions.

The airspace in and around Washington, D.C., is more restricted than in any other part of the country, the FAA says. The restrictions cover the area within a 30-mile radius of Ronald Reagan Washington National Airport.

"If it is determined that an aircraft poses an imminent security threat, deadly force is possible," an FAA training course for pilots says.

Pilots who violate restricted airspace can face criminal penalties of up to $250,000 and jail time, whether or not the violation was intentional.

President Barack Obama was "briefly relocated" in July 2009 when a single-engine plane briefly flew into restricted airspace. Two F-16 fighters and two Coast Guard helicopters intercepted the plane. The owner of the Maryland airport where the plane landed said the pilot had had "a navigation mistake."

In 2015, a mail carrier from Florida violated the restricted airspace in a small, one-person gyrocopter. He landed on the Capitol lawn because he wanted to deliver letters to members of Congress, he told News4 after serving four months in federal prison.