SIERRA VISTA, Ariz. — The pilot has gone through his checklist and taxied his plane into position for takeoff.

He gets clearance from the control tower, throttles forward and — from the ground — guides his unmanned aircraft into the sky along the Mexican border to watch for drug traffickers and illegal immigrants, part of a bird’s-eye patrol that authorities hope to expand.

Four Predator B drones have become fixtures over Arizona since October 2006, and two more will join them soon, Juan Munoz-Torres, a Customs and Border Protection spokesman, said Wednesday.

Once those six are in place, the agency wants Congress to fund six drones along the Canadian border and six more on Florida’s Gulf Coast and the Caribbean, said Douglas Koupash, who heads Customs and Border Protection’s drone program.

“The challenge on the northern border is the vast wilderness and number of trees,” Koupash said. “We may have to work with our technology partners to see if we can adapt different types of sensors” to see through the forests.

“You’re talking about really, really vast spaces and our ability to get to some of the remote spaces efficiently,” Koupash said recently.

The Predator B’s used for these missions are unarmed civilian adaptations of missile-toting drones used by the U.S. military in Iraq and Afghanistan. Each weighs five tons, has a 66-foot wingspan and can fly virtually undetected at altitudes of up to 50,000 feet, said Pete McNall, deputy director for Customs’ unmanned aerial systems in the Southwest.

The border agency’s fully loaded, $10.5 million Predators carry long-range cameras, but even at night, operators using the drones’ radar imaging and infrared capabilities can light a target with a laser visible only through the night vision goggles of helicopter crews who intercept some of the border crossers.

“That’s like a little red finger from God coming down and saying, ‘Hey, there’s some guy under that tree right there.’ Very effective,” McNall said.

From October 2006 through Feb. 16, the drones had helped in the apprehension of 3,857 illegal immigrants and the seizure of more than nine tons of marijuana, according to the most recent statistics available. Those numbers don’t include apprehensions and seizures credited to different kinds of drones tested in Arizona in 2004 or to a Predator B that flew from October 2005 until it crashed the following April; the National Transportation Safety Board ruled pilot error as the likely cause.

Officials say intelligence gathering drives each flight, but critics question whether the aerial surveillance doesn’t abuse the privacy of American citizens.

Lee Tien, senior staff attorney with the Electronic Frontier Foundation in San Francisco, said the flights are likely to create “a civil liberties-free zone along the border.”

His organization has not received any complaints about Predators, but Tien said he assumed “that’s because they can’t see them or aren’t aware of them.”

“It’s Catch-22,” said Barry Steinhardt, director of the American Civil Liberties Union’s Technology and Liberty Project. “How can you tell if you’re being pictured if you can’t tell whether you’re being surveilled or not?”