The U.S. Border Patrol is moving to halt a revolving-door policy of sending migrants back to Mexico without any punishment.

The agency this month is overhauling its approach on migrants caught illegally crossing the 1,954-mile border that the United States shares with Mexico. Years of enormous growth at the federal agency in terms of staff and technology have helped drive down apprehensions of migrants to 40-year lows.

The number of agents since 2004 has more than doubled to 21,000. The Border Patrol has blanketed one-third of the border with fences and other physical barriers, and spent heavily on cameras, sensors and other gizmos. Major advances in fingerprinting technology have vastly improved intelligence on border-crossers. In the 2011 fiscal year, border agents made 327,577 apprehensions on the Mexican border, down 80 percent from more than 1.6 million in 2000. It was the Border Patrol's slowest year since 1971.

It's a far cry from just a few years ago. Older agents remember being so overmatched that they powerlessly watched migrants cross illegally, minutes after catching them and dropping them off at the nearest border crossing. Border Patrol Chief Mike Fisher, who joined the Border Patrol in 1987, recalls apprehending the same migrant 10 times in his eight-hour shift as a young agent.

The Border Patrol now feels it has enough of a handle to begin imposing more serious consequences on almost everyone it catches, from areas including Texas' Rio Grande Valley to San Diego. The "Consequence Delivery System" -- a key part of the Border Patrol's new national strategy to be announced within weeks -- relies largely on tools that have been rolled out over the last decade on parts of the border and expanded. It divides border crossers into seven categories, ranging from first-time offenders to people with criminal records.

Punishments vary by region but there is a common thread: Simply turning people around after taking their fingerprints is the choice of last resort. Some, including children and the medically ill, will still get a free pass by being turned around at the nearest border crossing, but they will be few and far between.

"What we want to be able to do is make that the exception and not necessarily the norm," Fisher told The Associated Press.

Consequences can be severe for detained migrants and expensive to American taxpayers, including felony prosecution or being taken to an unfamiliar border city hundreds of miles away to be sent back to Mexico. One tool used during summers in Arizona involves flying migrants to Mexico City, where they get one-way bus tickets to their hometowns. Another releases them to Mexican authorities for prosecution south of the border. One puts them on buses to return to Mexico in another border city that may be hundreds of miles away.

In the past, migrants caught in Douglas, Ariz., were given a bologna sandwich and orange juice before being taken back to Mexico at the same location on the same afternoon, Fisher said. Now, they may spend the night at an immigration detention facility near Phoenix and eventually return to Mexico through Del Rio, Texas, more than 800 miles away.

Those migrants are effectively cut off from the smugglers who helped them cross the border, whose typical fees have skyrocketed to between $3,200 and $3,500 and are increasingly demanding payment upfront instead of after crossing, Fisher said. At minimum, they will have to wait longer to try again as they raise money to pay another smuggler.

"What used to be hours and days is now being translated into days and weeks," said Fisher.

The new strategy was first introduced a year ago in the office at Tucson, Ariz., the patrol's busiest corridor for illegal crossings. Field supervisors ranked consequences on a scale from 1 to 5 using 15 different yardsticks, including the length of time since the person was last caught and per-hour cost for processing.

The longstanding practice of turning migrants straight around without any punishment, known as "voluntary returns," ranked least expensive -- and least effective.

Agents got color-coded, wallet-sized cards -- also made into posters at Border Patrol stations -- that tells them what to do with each category of offender. For first-time violators, prosecution is a good choice, with one-way flights to Mexico City also scoring high. For known smugglers, prosecution in Mexico is the top pick.

The Border Patrol has introduced many new tools in recent years without much consideration to whether a first-time violator merited different treatment than a repeat crosser.

"There really wasn't much thought other than, `Hey, the bus is outside, let's put the people we just finished processing on the bus and therefore wherever that bus is going, that's where they go,"' Fisher said.

Now, a first-time offender faces different treatment than one caught two or three times. A fourth-time violator faces other consequences.

The number of those who have been apprehended in the Tucson sector has plunged 80 percent since 2000, allowing the Border Patrol to spend more time and money on each of the roughly 260 migrants caught daily. George Allen, an assistant sector chief, said there are 188 seats on four daily buses to border cities in California and Texas. During summers, a daily flight to Mexico City has 146 seats.

Only about 10 percent of those apprehended now get "voluntary returns" in the Tucson sector, down from about 85 percent three years ago, said Rick Barlow, the sector chief. Most of those who are simply turned around are children, justified by the Border Patrol on humanitarian grounds.

Fisher acknowledged that the new strategy depends heavily on other agencies. Federal prosecutors must agree to take his cases. U.S. Immigration and Customs Enforcement must have enough beds in its detention facilities.

In Southern California, the U.S. attorney's office doesn't participate in a widely used Border Patrol program that prosecutes even first-time offenders with misdemeanors punishable by up to six months in custody, opting instead to pursue only felonies for the most egregious cases, including serial border-crossers and criminals.

Laura Duffy, the U.S. attorney in San Diego, said limited resources, including lack of jail space, force her to make choices.

"It has not been the practice (in California) to target and prosecute economic migrants who have no criminal histories, who are coming in to the United States to work or to be with their families," Duffy said. "We do target the individuals who are smuggling those individuals."

Fisher would like to refer more cases for prosecution south of the border, but the Mexican government can only prosecute smugglers: smuggling migrants is a crime in Mexico but there is nothing wrong about crossing illegally to the United States. It also said its resources were stretched on some parts of the border.

Criticism of the Border Patrol's new tactics is guaranteed to persist as the new strategy goes into effect at other locations. Some say immigration cases are overwhelming federal courts on the border at the expense of investigations into white-collar crime, public corruption and other serious threats. Others consider prison time for first-time offenders to be excessively harsh.

The Border Patrol also may be challenged when the U.S. economy recovers, creating jobs that may encourage more illegal crossings. Still, many believe heightened U.S. enforcement and an aging population in Mexico that is benefiting from a relatively stable economy will keep migrants away.

"We'll never see the numbers that we saw in the late 1990s and early 2000s," said Edward Alden, senior fellow at the Council on Foreign Relations.

Doris Meissner, who oversaw the Border Patrol as head of the former Immigration and Naturalization Service in the 1990s, said the new approach makes sense "on the face of it" but that it will be expensive. She also said it is unclear so far if it will be more effective at discouraging migrants from trying again.

"I do think the Border Patrol is finally at a point where it has sufficient resources that it can actually try some of these things," said Meissner, a senior fellow at the Migration Policy Institute.

Tucson, the only sector to have tried the new approach for a full year, has already tweaked its color-coded chart of punishments two or three times. Fisher said initial signs are promising, with the number of repeat crossers falling at a faster rate than before and faster than on other parts of the border.

"I'm not going to claim it was a direct effect, but it was enough to say it has merit," he said.