A U.S. Customs and Border Patrol drone aircraft lifts off Sept. 24 at Fort Huachuca in Sierra Vista, Ariz. (Matt York/AP)

U.S. drones deployed along the borders are grounded most of the time, cost far more than initially estimated and help to apprehend only a tiny number of people trying to cross illegally, according to a federal audit released Tuesday.

In a report that could undermine political support for using more drones to secure the nation’s borders, the Department of Homeland Security’s inspector general found “little or no evidence” that the fleet had met expectations or was effective in conducting surveillance.

U.S. Customs and Border Protection has been flying surveillance drones for nearly a decade, launching them from bases in Texas, Florida, North Dakota and Arizona. The agency has nine of the Predator B model — a modified version of the MQ-9 Reaper drone flown by the Air Force — and has plans to more than double the size of its drone fleet to 24 as part of a $443 million expansion.

The inspector general, however, questioned whether those plans make any sense or would be cost-effective.

In an audit of the fleet’s operations during fiscal 2013, the inspector general calculated that it cost $12,255 per flight hour to operate the drones, five times as much as Customs and Border Protection had estimated.

Although the agency planned to fly four drone patrols a day — each for an average of 16 hours — the aircraft were in the air for less than a quarter of that time, the audit showed. Bad weather and a lack of personnel and spare parts hindered operations, it concluded.

“The unmanned aircraft are not meeting flight hour goals,” the auditors wrote, adding more broadly that Customs and Border Protection “cannot demonstrate how much the program has improved border security.”

As evidence, the report cited statistics showing that of the 120,939 illegal border crossers apprehended in Arizona during 2013, fewer than 2 percent were caught with the help of drones providing aerial surveillance.

In Texas and the Rio Grande Valley, less than one-tenth of 1 percent of border-crossing apprehensions were attributed to drone detection.

The findings echo earlier audits by the inspector general of the domestic drone program but could carry extra weight as Congress considers whether to spend more on drone surveillance to secure the borders as part of immigration legislation.

In a written response to the audit, Eugene Schied, an assistant commissioner with Customs and Border Protection, disputed the characterization in the findings. The drone program, he said, “has achieved or exceeded all relevant performance expectations.”

Schied accused the inspector general of cherry-picking statistics and ignoring information that makes the drones appear more effective. For instance, Schied said, drones “directly contributed” to the seizure of almost 50,000 pounds of marijuana, worth an estimated $122 million, along the Southwest border in 2013.

Customs and Border Protection dismissed suggestions that a major expansion of its drone fleet would occur anytime soon. Although plans to fly as many as two dozen drones were authorized years ago, Schied said the department did not have the money to follow through and that “there is no intent at this time” to operate more than 10 of the aircraft.