WASHINGTON — A small drone flying at low altitude crashed into the White House complex before dawn Monday, the Secret Service said. President Barack Obama was not at home and the White House said it did not pose a security threat.

The crash prompted an immediate lockdown of the White House grounds until officials could examine the drone. The Secret Service said the 2-foot-long device was a quadcopter — a small, unmanned aircraft that is lifted by four propellers.

“An investigation is underway to determine the origin of this commercially available device, motive, and to identify suspects,” said Secret Service spokesman Brian Leary. He said the drone crashed on the southeast side of the White House complex just after 3 a.m.

Many small quadcopters are essentially sophisticated toys that can also be useful for commercial operations like aerial photography and inspection. 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. More elaborate models sells for thousands.