The Homeland Security Department (search) will not gain authority to order shootdowns of threatening aircraft over the nation's capital, officials said Tuesday, eliminating possible confusion between its role and the Pentagon's during emergencies.

Questions over whether Homeland Security could take down planes surfaced after a single-engine Cessna (search) mistakenly wandered into restricted airspace on May 11, coming within three miles of the White House. Currently, the Defense Department is the only agency authorized to shoot down aircraft around Washington.

Defense Secretary Donald Rumsfeld (search) and Homeland Security Secretary Michael Chertoff have not yet signed off on an agreement clarifying the Pentagon's authority to order shootdowns, Assistant Defense Secretary Paul McHale said Tuesday.

But "I can tell you in principle that both Cabinet officials believe that when it comes to a decision to shoot down an aircraft, only one person should have that authority to avoid an unintended conflict of judgment," McHale said. "And that person should be the secretary of defense, accountable to the president."

"It's a basic military principle to achieve unity of command — particularly in a life-and-death situation," McHale said.

Homeland Security aircraft will continue to respond to airborne threats above Washington. But Rear Adm. Timothy S. Sullivan, Chertoff's top military adviser, said that under the forthcoming agreement, they could only shoot down a plane if ordered by the North American Aerospace Defense Command, an arm of the Pentagon.

The Cessna set off a false alarm when it broke a 15-mile radius of highly restricted airspace around downtown Washington, prompting evacuations at the White House, the Capitol and other federal buildings. Homeland Security helicopters joined F-16 fighter jets to divert the Cessna away from Washington just as national security officials discussed shooting it down.

Following that scare, some Homeland Security officials suggested giving airspace protection missions in the capital region to the Coast Guard, which is part of the Homeland Security Department.

But other officials, as well as lawmakers, have questioned whether the Coast Guard is being stretched too thin to take on a new assignment while it grapples to fulfill its traditional rescue and maritime safety missions.

Currently, helicopters from U.S. Customs and Border Protection — another branch of the Homeland Security Department — respond to airborne threats above Washington.

Whether Customs or the Coast Guard will ultimately be in charge of protecting the region's airspace has not yet been decided, said Homeland Security spokesman Russ Knocke.