Lack of coordination between the U.S. government and some Indian tribal authorities has undermined border security in some areas, according to a new government report released as senators debate how to address border security in a new immigration reform deal.

“Eighty-six miles of the northern border and 68 miles of the southwest border are on 13 Indian reservations, many of which are vulnerable to illicit cross-border threat activity, such as drugs, weapons, and human smuggling, according to DHS officials,” the Government Accountability Office’s Rebecca Gambler wrote to Sen. Jon Tester, D-Mont., in a letter accompanying the new report.

The report explains one reason for weak border security along the reservations:

Further, according to Border Patrol officials, in some cases, coordination challenges with tribes have affected the Border Patrol’s ability to patrol and monitor the border so as to prevent and detect illegal immigration and smuggling. Border Patrol officials from three of the seven Border Patrol sectors and 5 of the 10 stations we contacted reported coordination challenges related to understanding and collaborating with tribes within tribal government rules. Specifically, officials from two sectors that include Indian reservations and corresponding stations reported coordination challenges related to tribal government rules that hindered law enforcement in working together to secure the border.

Law enforcement coordination with the tribes has improved somewhat. “Officials from one of the tribes in our review also reported that DHS and the Border Patrol at both the national and local levels are more sensitive to tribal concerns now than in the past and that the Border Patrol is willing to work with tribal law enforcement in sharing intelligence and keeping the lines of communication open,” the report says.

The report comes as a group of eight bipartisan senators tries to negotiate a comprehensive immigration reform bill, with the issue of increased border security a key sticking point.

Tester, who requested that the GAO conduct this study, played a key role in blocking the 2007 immigration reform proposal by getting rid of language in the bill that would have required employers to use certain documents to check the eligibility of their employers.

“If by fighting to keep government out of people’s private lives, Max Baucus and I stopped the senate from passing this flawed immigration bill, then this was a real victory for Montana and the American people,” Tester said in a statement at the time.