A U.S. Border Patrol agent surveys the U.S.-Mexico border in southern Arizona in 2013. Reuters

The Tucson (Arizona) Police Department has been detaining people at traffic stops for longer than necessary in order to check their immigration status, according to a letter sent by the American Civil Liberties Union of Arizona to the department Monday. The lengthy stops, the letter complains, go beyond requirements in the state’s SB 1070 law, which asks officers to make a “reasonable attempt” to determine immigration status, the ACLU alleges.

The letter says roughly 85 out of 110 cases the ACLU reviewed in records from June 2014 to December 2015 show officers held people longer than necessary to check immigration status, and in some cases coordinated directly with Border Patrol during the stop. Those records, which confirmed anecdotal evidence heard by the organization, were obtained through open government requests, ACLU staff attorney James Lyall wrote in the letter to Tucson police.

The stops “reflect a fundamental misunderstanding of the Fourth Amendment’s prohibition on prolonging stops and limits on the authority of local police to enforce immigration laws,” the letter states.

A separate letter was sent to Homeland Security Secretary Jeh Johnson saying Border Patrol had played a part in the lengthy traffic stops.

Border Patrol involvement shows “disregard for DHS enforcement priorities and contradicts the Obama administration’s commitment… to limit the involvement of federal immigration officials in traffic stops by Arizona law enforcement,” the letter says in part.

Foreign-Born Population in Arizona - 2010 | MooseRoots

The law cited in the letter, SB 1070, became a hotly contested piece of legislation in 2010, garnering national attention and inspiring copycat legislation in at least five states: Alabama, Georgia, Indiana, South Carolina and Utah. The ACLU challenged the bill, which requires police to determine the immigration status of anyone arrested or detained if there is “reasonable suspicion” that they are in the country illegally.

A challenge in the United States Supreme Court upheld most of the law; however, some states — like Mississippi, Virginia, Kansas and others — have turned away from the model that many view as discriminatory. In addition to that controversy, the American Immigration Council estimated that enforcement of the law could reach into the hundreds of millions of dollars for the state.