The U.S. government has suspended a $100 million voluntary air deportation program targeting Arizona undocumented immigrants because there aren't enough people crossing the border to fill flights, the Arizona Republic reports.

The Mexican Interior Repatriation Program, which had been operating every summer since 2004 on an agreement between Homeland Security officials and the Mexican government, was designed to discourage immigrants from entering the U.S. through Arizona's harsh desert climate. Under the program, funded by the U.S., once caught, immigrants were flown from Tucson back to Mexico City -- 1,100 miles away -- and then given bus tickets back to their hometowns.

"What we know doesn't work is, arresting somebody, putting them back across the border in the same place where you just caught them, so that they can come back again," former U.S. Attorney Pete Nuñez told KNSD-TV.

Since its inception, a total of 125,164 Mexican immigrants have voluntarily been flown back, but only 8,893 were deported last year, a decline that underscored an overall decline in the number of successful crossings, which has been attributed to both more sophisticated border surveillance and a tougher employment climate in the U.S. And this year, there have not been any flights at all.

The Associated Press reported that the program cost $90.6 million a year and targeted first-time offenders and families caught looking for work. With the flights now suspended, authorities are now either busing immigrants to cities like Del Rio, Texas or San Diego and Calexico in California, or detaining them in jail in Tucson. A separate involuntary flight program, the Interior Removal Initiative, is slated to start next year.

According to The Republic, the program had fallen under scrutiny after records showed that between 2008 and 2010, between 6 and 12 percent of people flown out were caught trying to cross the border again. A review by the Government Accounting Office (PDF) two years ago also criticized immigrations officials for failing to provide an accurate measure of how well the program was working.

With the number of flights declining, the Mexican government rejected a proposal to boost the numbers by including criminal deportees with families. U.S. officials are negotiating to reinstate the voluntary flight program, but it would not start again until next year at the earliest.

"Right off the bat, I can tell you that Mexico was not going to allow, nor will it ever allow, that kind of repatriation, which puts families' safety at risk," a Mexican consulate official told the AP.

Update: KNSD's story on the program's suspension, aired Tuesday morning, can be seen below.

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