WASHINGTON — Although California is home to some of the nation's biggest drone manufacturers, the state was passed over Monday when the federal government picked six sites across the nation for testing the use of robotic aircraft in U.S. airspace.

The testing is designed to help the Federal Aviation Administration meet a 2012 congressional mandate to open the skies to remotely piloted aircraft, or drones, by 2015.

Disappointed California officials were at a loss to explain their failure to land a test site, though some suggested the state didn't do enough to win in the fierce nationwide competition. The state lost out to Alaska, Nevada, New York, North Dakota, Virginia and — adding salt to the wound — longtime rival Texas.

California was the only state with two groups submitting bids — one based in Ventura County and the other in Kern County.

"How California was left off the list, I haven't got a clue," said Bill Buratto, who, as president and chief executive of the Ventura County Economic Development Assn. helped pitch a bid for a test site in California. "It would seem to me that the FAA would look favorably on California."

California was thought to be a front-runner, given its rich aerospace history and the presence of major military drone makers, such as AeroVironment Inc. of Monrovia, General Atomics Aeronautical Systems Inc. of Poway, and Lockheed Martin Corp. and Northrop Grumman Corp., both of which build drones in Palmdale. Many of the military's newest drones take to the skies for the first time at Edwards Air Force Base in the Mojave Desert.

The FAA has estimated that 7,500 small drones could be aloft within five years, with industry officials predicting they could be used for such things as spotting wildfires, scouting film locations and delivering pizza.

In a conference call with reporters, FAA Administrator Michael Huerta said the agency considered a number of factors such as geographic and climatic diversity, availability of ground infrastructure, the types of proposed research and the volume of air traffic near the test sites.

Asked about the decision to reject the California proposals, he declined to say what was lacking in the losing bids. He said there would be no comment from the government until FAA officials can debrief the unsuccessful bidders.

"What we were really looking for was how do we select six that give us the broadest base of different airspace configurations, different traffic configurations and different climates," Huerta said.

Although the test site designation offered no federal money, groups in 24 states competed for it. The groups saw it as an opportunity to generate jobs from a burgeoning industry — and a matter of prestige.

Some states allocated money to promote their bids.

Brook Taylor, spokesman for Gov. Jerry Brown's Office of Business and Economic Development, said the governor's military council offered its strong support and the state worked directly with both applicants to strengthen their bids.

Winners were ecstatic. Sen. Mark Begich (D-Alaska) said the designation would "put our state on the world map when it comes to this exciting emerging technology.'' Nevada Gov. Brian Sandoval, in a tweet, called the selection a "historic moment" for his state.

The competition was smart on the government's part, because it raised awareness about the emergence of commercial drones and built coalitions between state and local politicians to push the technology forward, said Peter W. Singer, author of "Wired for War," a book about robotic warfare.

"It's a huge step for the advancement of the industry," Singer said. "It makes the future of the technology and the political, legal and ethical questions all the more real."

Drones, which have played an increasing role in military combat around the world, have also begun to take on new commercial roles, often going where it's too dangerous for a pilot in a cockpit.

They have helped measure radiation in the wake of the meltdown at Japan's Fukushima Daiichi nuclear power plant, penetrated the eyes of hurricanes to gather scientific data and helped firefighters see hot spots during wildfires. The FAA has also allowed limited surveillance uses for a few police agencies.

Currently, drones are not allowed to fly in the U.S. except with special permission from the FAA. The agency said there are 545 active certifications for such flights, many involving educational and scientific uses.

As drone technology becomes more advanced, demand has increased from police agencies and others for using drones. Oil and gas companies want to utilize them to keep an eye on their pipelines. Farmers in Japan already use small drones to spray their crops with pesticides. The idea of using drones as transport vehicles has been discussed as a way to deliver tacos and Domino's pizza.