Illegal immigration is without question the most debated and divisive issue in Maricopa County. And no one is more passionate about it than Sheriff Joe Arpaio. Over the past two years, the sheriff has invested millions of dollars and much political energy in a highly publicized crackdown on illegal immigrants, one of the most aggressive anti-illegal immigration efforts in U.S. history.

Six months ago, the Tribune began an investigative project to answer three basic questions about Arpaio’s illegal immigration enforcement effort: How does it work? What is it costing the taxpayers? And what is the effect on other aspects of his agency — and public safety in general — if his focus has become so heavily on illegal immigration?

This five-part series marks the first time anyone has examined Maricopa County Sheriff’s Office operations in such depth. Sheriff’s officials were uncharacteristically responsive to public records requests, saying, at first, they were happy to show off the enforcement effort. Arpaio and his top deputies made enforcement files, budget numbers and numerous other records available to the Tribune. That hasn’t always been the case with the agency, and one story in this series discusses some ways the agency tried to block county auditors and the reporters.

Moreover, several top-ranking officers candidly discussed their policies and their concerns about the toll that is being taken on their operations as the cash-strapped agency tries to serve an area bigger than New Jersey and take on illegal immigration enforcement, too.

Tribune writers Ryan Gabrielson and Paul Giblin began by reviewing MCSO case files from 2006, when Arpaio’s operation began, through 2007. Using thousands of pages of unredacted records provided by the sheriff’s office, they built a database of criminal immigration arrests that includes in-depth information on the immigrants and well as the enforcement effort. The database is unique; not even the sheriff’s office has compiled such a systematic examination of the human smuggling enforcement operation.

Using other records obtained from MCSO, the state, the county, the federal government and cities that MCSO contracts with, the reporters were able to explore staffing levels and personnel assignments, budget and payroll information, response times, arrest rates and the status of MCSO operations throughout the county. They examined interagency e-mails and other correspondence, state and federal grant documents and the rules governing the so-called 287(g) authority that Immigration and Customs Enforcement extends to local agencies that agree to follow strict guidelines prohibiting racial profiling, among other things.

Beyond the facts and figures in public records, human sources were also vital to this project. Gabrielson and Giblin conducted nearly 100 interviews with deputies, federal agents, lawmakers, county and town officials, crime victims, local police chiefs and immigration experts. A number of people spoke about their concerns that law enforcement services in their communities have fallen off since the illegal immigration effort began.

The sheriff’s office also allowed the reporters and a photographer to ride along with immigration patrols near Wickenburg and with deputies on an urban “immigration sweep” in Phoenix.

The searchable database of criminal immigration cases is available online. We’ve also posted many of the county, state and federal documents used in this series as well as interactive graphics and maps and several videos, including Joe Arpaio talking at length about his immigration operations.