across time. In the mid-2000s, when USBP had fewer agents and the booming U.S. economy encouraged high levels of unauthorized entry, recordkeeping on apprehensions may have been incomplete. Additionally, the expansion of USBP resources has been uneven across the nine sectors of the U.S.–Mexico border (Borger et al., in press), which complicates spatial comparisons of administrative data.

Beginning in fiscal year 1999, USBP created an electronic record of each apprehension made by a USBP agent. Table 5-1 is a partial list of the variables contained in the ENFORCE database, in which USBP, OFO, and ICE apprehensions data are recorded. Individual USBP apprehension records contain demographic information on the person apprehended, including gender, date of birth, country of origin, and (if a Mexican national) state of birth. The records also include an event number—which permits linking of individuals apprehended together—and information on when and where the apprehension took place, including the nearest port of entry and the USBP station, district, and sector of the arresting officer. A fingerprint identification number (IDFINS), which since fiscal year 2000 has been based on all 10 fingers, essentially identifies an individual and enables one to “tag” migrants who are apprehended more than once. On days (or more precisely, evenings) when apprehensions run at high levels, individual USBP stations may not have the personnel to fingerprint or interview all

TABLE 5-1 Variables in ENFORCE Database

U.S. Border Patrol Data



Event number


Date of apprehension


Site of apprehension


Nearest port of entry


Border Patrol sector




Time in U.S.


Arrest method


Country of citizenship


Country of residence


Country of birth


State of residence (Mexico only)


State of birth (Mexico only)


Date of birth




Marital status


SOURCE: Office of Immigration Statistics, U.S. Department of Homeland Security (personal communication).

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