WASHINGTON — The government says apprehensions of people for federal immigration violations have dropped to the lowest level in 40 years, reflecting a decline in the northbound traffic of undocumented immigrants from Mexico.

At the same time, the number of suspects booked by the U.S. Marshals Service for criminal immigration offenses has gone up dramatically, a function of tougher law enforcement on the U.S. side of the border.

In a report released Wednesday, the U.S. Bureau of Justice Statistics said the number of immigration-related apprehensions has steadily declined, peaking at 1.8 million in 2000 but dropping to 516,992 in 2010 – the lowest level since 1972.

"The U.S. economic slump, more economic stability in some Mexican areas, increased U.S. enforcement on the border and in the interior plus the drop in the total fertility rate in Mexico are likely among the reasons for this drop," said University of Texas sociology professor Nestor P. Rodriguez.

"On the other hand, arrests by federal authorities for immigration offenses is rising, no doubt partly due to greater enforcement efforts," said Rodriguez, who has spent decades studying Latino migration and the impact of U.S. immigration policies on immigrant communities.

Suspects arrested by the Marshals Service for federal criminal immigration offenses increased from 8,777 in 1994 to 82,438 in 2010.

In a seven-year span ending in 2010, the number of border patrol officers nearly doubled, from 10,819 to 20,558, the study says.

The number of Mexican citizens serving a federal prison term for an immigration offense increased from 2,074 in 1994 to 17,720 in 2010. Nine out of 10 immigration offenders in federal prison were convicted of illegal re-entry or illegal entry offenses. Ten percent were convicted of alien smuggling.

More than eight out of 10 deportable aliens in 2010 were citizens of Mexico. An increasing share of deportable aliens are coming from countries in Central America, 12 percent in 2010, up from 3 percent in 2002.

In a report released Wednesday, the U.S. Bureau of Justice Statistics said the number of immigration-related apprehensions has steadily declined, peaking at 1.8 million in 2000 but dropping to 516,992 in 2010 – the lowest level since 1972.

"The U.S. economic slump, more economic stability in some Mexican areas, increased U.S. enforcement on the border and in the interior plus the drop in the total fertility rate in Mexico are likely among the reasons for this drop," said University of Texas sociology professor Nestor P. Rodriguez.

"On the other hand, arrests by federal authorities for immigration offenses is rising, no doubt partly due to greater enforcement efforts," said Rodriguez, who has spent decades studying Latino migration and the impact of U.S. immigration policies on immigrant communities.

Suspects arrested by the Marshals Service for federal criminal immigration offenses increased from 8,777 in 1994 to 82,438 in 2010.

In a seven-year span ending in 2010, the number of border patrol officers nearly doubled, from 10,819 to 20,558, the study says.

The number of Mexican citizens serving a federal prison term for an immigration offense increased from 2,074 in 1994 to 17,720 in 2010. Nine out of 10 immigration offenders in federal prison were convicted of illegal re-entry or illegal entry offenses. Ten percent were convicted of alien smuggling.

More than eight out of 10 deportable aliens in 2010 were citizens of Mexico. An increasing share of deportable aliens are coming from countries in Central America, 12 percent in 2010, up from 3 percent in 2002.

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  • Gary Mead, executive associate director for U.S. Immigration and Customs Enforcement shows clothing and personal items inmates will receive at a new civil detention facility for low-risk inmates in Karnes City, Texas, on Tuesday, March 13, 2012. Federal officials are holding up the new facility as the centerpiece of an initiative to treat those facing immigration violation charges more humanely after lawsuits filed in past years. (AP Photo/Will Weissert)

  • A guard holds open a door to the barber shop at a new civil detention facility for low-risk inmates in Karnes City, Texas, on Tuesday, March 13, 2012. Federal officials are holding up the new facility as the centerpiece of an initiative to treat those facing immigration violation charges more humanely after lawsuits filed in past years. (AP Photo/Will Weissert)

  • A guard walks by rooms at a new civil detention facility for low-risk inmates in Karnes City, Texas, on Tuesday, March 13, 2012. Federal officials are holding up the new facility as the centerpiece of an initiative to treat those facing immigration violation charges more humanely after lawsuits filed in past years. (AP Photo/Will Weissert)

  • Gary Mead, executive associate director for U.S. Immigration and Customs Enforcement stands by a soccer field at a new civil detention facility for low-risk inmates in Karnes City, Texas, on Tuesday, March 13, 2012. Federal officials are holding up the new facility as the centerpiece of an initiative to treat those facing immigration violation charges more humanely after lawsuits filed in past years. (AP Photo/Will Weissert)

  • Joe Arpaio

    FILE - In this Feb. 4, 2009 file photo, Maricopa County Sheriff Joe Arpaio, left, orders approximately 200 convicted undocumented immigrants handcuffed together and moved into a separate area of Tent City, for incarceration until their sentences are served and they are deported to their home countries, in Phoenix. The Homeland Security Department says it will use 50 immigration agents to screen jail inmates in Arizona

  • FILE - In this Wednesday, Feb. 4, 2009 file photo, approximately 200 convicted undocumented immigrants are handcuffed together and moved into a separate area of Tent City, by order of Maricopa County Sheriff Joe Arpaio, for incarceration until their sentences are served and they are deported to their home countries, in Phoenix. The Homeland Security Department says it will use 50 immigration agents to screen jail inmates in Arizona