Updated

The number of U.S. border apprehensions dropped for a third consecutive year, falling to a level not seen since 1973, new government data show.

The U.S. Border Patrol last year had 724,000 apprehensions, falling more than 17 percent from 2007, according to the Department of Homeland Security's Office of Immigration Statistics.

Ninety-seven percent were on the southwest border, and 91 percent of those caught were Mexican. The report released Tuesday cited the U.S. recession and tougher border security as possible factors.

Border Patrol apprehensions peaked in 1986, with nearly 1.7 million. But the data are a crude measure of immigration since they only address those stopped.

Jeffrey Passel, senior demographer at the Pew Hispanic Center, said the data appear to follow other reports that showed steep declines in Mexican immigration. The main reason Mexicans emigrate to the U.S. is for jobs, he said, "and there aren't any."

While enhanced border security may be a factor, it is difficult to tell because the decline corresponds so closely with the economy, he said. The real test will come when the economy bounces back.

The Border Patrol grew to more than 18,000 agents by the end of 2008, more than doubling during the presidency of George W. Bush.

There were 12.7 million Mexican immigrants living in the United States in 2008, according to the Pew Hispanic Center. About 55 percent of those were here illegally.