Washington ( CNN ) -- Lenient enforcement of a law requiring U.S. citizens to have passports when re-entering the country at land border crossings has heightened the risk that an imposter might get in , according to a government report released Monday . Under the law , which took effect in June 2009 , U.S. citizens must show passports or some other authorized travel documents like a military ID when returning to the United States . Those who do n't are supposed to undergo further screening to confirm their citizenship . But , during a phase-in period that now has stretched over 18 months , very few travelers have been referred to secondary screening , the report from the Homeland Security department 's inspector general 's office found . That assertion -- the study concluded -- `` increases the risk that someone could enter the U.S. under false pretense of citizenship . '' The federal Customs and Border Protection agency , though , contends that the program is working , adding that it believes it is better to encourage compliance gradually then to enforce it right away . The inspector general 's report is one of the first to look at compliance with the new passport law , which is known as the Western Hemisphere Travel Initiative . The law impacts all U.S. , Canadian and Bermudian citizens , who previously were able to enter the United States at checkpoints along the Canadian and Mexican borders without passports . On a typical day , some 700,000 people arrive at U.S. land borders , trying to get into the country . But every day , authorities say , some 4 percent -- or 28,000 people -- do not have the required documents . In an effort to implement the law without clogging international borders , Customs and Border Protection instituted a phase that it called `` informed compliance '' to transition the law into effect over time . It sought compliance through a carrot-and-stick approach , by promising the benefit of speedier travel and punishment of secondary screening and travel delays . The results have been mixed . Compliance has been greatest along the northern border , where 98 percent of travelers had the appropriate documents , the inspector general 's report says . Along the southern border , compliance has been spottier , with an average of 93 percent of travelers having required documents . Compliance is lowest at Texas border crossings , where 91 percent of travelers had everything they needed . The report suggests that the threat of slower travel has not led to higher compliance , noting that compliance rates have leveled off over time . The inspector general says the non-compliance rate could spell trouble when Customs and Border Protection seeks `` full compliance '' with the law , saying the latter agency does not have the resources to process large numbers of non-compliant travelers through secondary screening . As an example , it said the workload among secondary inspectors at the El Paso , Texas , border crossing could increase 125 percent with `` full compliance '' of the law . As the San Ysidro crossing in California -- the busiest land border crossing in the United States -- officials have only eight computer workstations at which to process an estimated 1,834 secondary inspections per day . `` Processing this number of travelers with these few computer workstations may cause considerable traveler delays , '' the report says . In a letter to the inspector general , Customs and Border Protection officials took issue with the statement that the passport law 's full benefits will not be felt until the final `` full enforcement '' phase takes effect . The agency said the program is working , adding that it believes it is better to encourage compliance then to enforce it at this time . `` Travelers are never admitted until identity , citizenship and admissibility are established to the satisfaction of the inspecting officer , '' the agency said . `` In the event that a traveler does not possess a ( passport or other authorized document ) , the inspecting officer will use all available documentary and system information , as well as oral questioning and inspection techniques , to establish identity and citizenship . ''
A Homeland Security department IG report looks at implementation of a new immigration law . The study finds that many people get in the country , by land , without required permits . Compliance rates are much higher along the Canadian border than the Mexican border . The border patrol agency says its built - in compliance program is working .