WASHINGTON >> A small drone flying low to the ground crashed onto the White House grounds before dawn Monday, triggering a major emergency response and raising fresh questions about security at the presidential mansion.

Although President Barack Obama was not at home, the security breach prompted a lockdown of the entire complex until officials could examine the drone. The White House later said the drone did not pose a threat.

The Secret Service launched an immediate investigation into the origins of the drone, which crashed on the southeast side of the White House grounds just after 3 a.m. Secret Service spokesman Brian Leary said officials were also working to identify any suspects and determine what their motive might have been.

The device was described as a two-foot-long quadcopter — an unmanned aircraft that is lifted by four propellers. Many small quadcopters are essentially sophisticated toys that can also be useful for commercial operations like aerial photography and inspections. Often weighing only a few pounds, they sell for as little as a few hundred dollars or less, and were popular Christmas gifts last year.

The Secret Service said the drone discovered Monday was of the commercially available variety.

The president and first lady Michelle Obama are traveling in India and were not present for the incident, but their daughters, Sasha and Malia, may have been at home. White House officials declined to comment on the daughters’ whereabouts Monday, but ahead of the president’s trip aides had said the daughters would remain in Washington so as not to miss school.

"The early indications are that it does not pose any sort of ongoing threat to anybody at the White House," said presidential spokesman Josh Earnest.

Still, the incident was likely to reinvigorate a long-running public debate about the use of commercial drones in U.S. skies — as well as White House security. At the urging of the drone industry, the Obama administration is on the verge of proposing rules for drone operations that would replace an existing ban on most commercial flights.

Although remote-controlled airplanes and related toys have been available for decades, the recent proliferation of inexpensive drones has prompted growing fears about potential collisions with traditional aircraft. Technological advances have also made it easier to equip drones with advanced capabilities such as cameras, raising privacy issues as well as concerns that such devices could carry weapons.

White House aides could not recall any similar incidents occurring at the complex.

Police, fire and other emergency vehicles swarmed the White House just after the crash, with several clustered near the southeast entrance to the grounds. The White House was dark and the entire perimeter was on lockdown until around 5 a.m., when those who work there were allowed inside.

After daylight, more than a dozen Secret Service officers fanned out in a search across the White House lawn as snow began to fall. They peered down in the grass and used flashlights to look through the large bushes that line the driveway on the south side of the mansion.

It was not immediately clear that the Secret Service could have done anything to prevent the incident. Yet the episode joins a string of recent security breaches at the White House that have fueled questions about the agency’s effectiveness and ability to protect the president.

Four high-ranking executives were reassigned this month, and former Director Julia Pierson was forced to resign last year after a Texas man armed with a knife was able to get over a White House fence and run into the executive mansion before being subdued.

An independent panel that investigated the agency’s leadership and practices after that September incident, and the disclosure of a previously unreported security breach, recommended hiring a new director from outside.

That report was the second critical review of the agency responsible for protecting the president. In November, the Homeland Security Department, which oversees the Secret Service, released an internal investigation about the fence-scaling incident that found poor training and staffing and a series of missteps led to the breach.

Homeland Security investigators found, among other things, that uniformed agents patrolling the White House grounds the night of Sept. 19 mistakenly assumed that thick bushes near the mansion’s front door would stop the intruder.

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Associated Press writer Joan Lowy contributed to this report.