In late February, Border Patrol agents apprehended and deported Roberto Robledo Sandoval after finding him with others inside a drop house in Mesa.

Robledo Sandoval, 45, called the experience — armed men kept them in the house waiting for family members in Mexico to wire more money — the worst nightmare of his life.

Nonetheless, after Border Patrol agents dropped him off the border in Nogales, he found another coyote — a people smuggler — and tried again the next day. A couple of days later, while walking in the Altar Valley southwest of Tucson, Border Patrol agents caught him again.

His story is a common one among the estimated 500,000 illegal entrants who make their way into the country each year, said Princeton professor Douglas Massey. He said research suggests that apprehensions don't stop migrants but rather force them to try repeatedly until they make it.

Yet, apprehension numbers kept by the Border Patrol don't reflect this reality.

Under the agency's guidelines, Robledo Sandoval would count as two apprehensions because the agency counts the event of each apprehension, not the number of people apprehended.

Internally, the Border Patrol tracks how many times each illegal entrant is caught, said Shannon Stevens, a spokeswoman with the Border Patrol's Tucson Sector. However, the agency doesn't release that number and didn't have an estimate on how many detainees have been caught previously.

The agency views the 11 percent decrease in apprehensions from fiscal year 2004 to fiscal 2005 and the 8 percent decrease so far in fiscal year 2006 in the Tucson Sector as an indicator of declining traffic in the sector and validation of the increased agents and technology devoted to the sector.

"I don't think it's due to the fact that we aren't catching as many," Stevens said. "I just don't think as many are crossing here."

But professors who study migration patterns such as Massey, Trinity University's David Spener and the Migration Policy Institute's Demetrios Papademtriou doubt that Border Patrol actions change the actual number of illegal entrants who make it into the United States.

Massey said the probability that a man or woman would leave Mexico to migrate illegally into the United States has fluctuated at around 1 percent for the past two decades despite the increase in Border Patrol agents and budget. Nearly all make it, Massey said.

"Almost everybody gets in," Massey said. "It's just a matter of how many times it takes."

The number of Border Patrol agents and agency budget has been on the rise since 1992. The number of agents has more than doubled nationally from 4,139 in 1992 to 11,384 in 2006. When adjusted for inflation, the agency's budget has increased 220 percent since 1992 to $1.413 billion in 2005.

"There is virtually nothing one can infer about the volume of illegal migration from the number of apprehensions," said Massey, the author of "Beyond Smoke and Mirrors: Mexican Immigration in the Age of Economic Integration."

Spener, a sociologist at Trinity University in San Antonio, Texas, who studies U.S.-Mexican border relations, said that when border-wide apprehension numbers bulge in one sector they usually drop in nearby sectors or vice versa. But, like a balloon, the overall numbers remain nearly the same, Spener said.

Current numbers support his theory.

As Tucson Sector apprehensions declined by 10 percent in fiscal year 2005, apprehensions increased by 41 percent in the Yuma Sector to the west and by 18 percent in the El Paso Sector to the East. That trend continues so far this year as well.

Borderwide apprehensions for 2005 were only 5 percent higher than they were in 1984. In between, though, apprehensions peaked as high as 1.6 million in 1986 and in 2000 and below 1 million in 1988-1989 and again in 2002-2003.

Border Patrol officials recognize the funneling or bulging theory.

"If it makes it harder for them to cross, we are going to see a decrease because they are going to cross somewhere else," said Stevens, about increased resources in the Tucson Sector.

Massey and Spener said the Border Patrol deterrent programs, which started in El Paso in 1993 with Operation Hold the Line, Operation Gatekeeper in San Diego in 1994, and Operation Safeguard in Arizona in 1995, have done a better job at keeping illegal entrants in the country than out.

As it became more difficult to travel back and forth, married men brought their families and single men got married and had kids, Spener said.

"The longer they are away from their town or village in Mexico, there is less to go back to in many ways," Spener said. "So, it really encourages settlement."

Papademtriou said it would be incorrect to call this locking-in effect an "unintended consequence" because it's been happening for a decade, enough time for legislators to see the results. He said legislators in Washington "throw money" at the border as an easy way out of a problem they can't control.

"Clearly, we know this is happening, and we keep doing more of the same so clearly we must be comfortable with the consequences," said Papademtriou, president of the Migration Policy Institute, a Washington-based think tank that studies international migration

As he sat in the Nogales office of Grupo Beta, Mexico's special force for protecting migrants, Robledo Sandoval was already contemplating another attempt.

The reason: Back home in Mexico he can only make $80 a week as a painter, a fraction of what he can make in the United States.

If he tries and gets caught again, he'll add another digit to the ambiguous apprehension numbers.

See tally of apprehensions, seizures. Page B2

● Contact Brady McCombs at 573-4213 or bmccombs@