On Oct. 6, 1999,  The U.S. Supreme Court refused to hear an appeal of a landmark decision by the 10th Circuit Court of Appeals, confirming that state and local law enforcement officials are free to arrest illegal aliens to the full extent permitted by state law.

The ruling in United States vs. Ontoniel Vasquez-Alvarez strikes down the widespread urban myth that local police have no power to arrest illegal aliens. Vasquez-Alvarez, an illegal alien with two California felony convictions and three prior deportations to Mexico, claimed his 1998 arrest by Edmond Oklahoma police was illegal under federal law, because local police could only arrest him for immigration crimes if the INS first confirmed that he was an aggravated felon with a prior conviction for illegal reentry. The Edmond police were unaware of his record.

The 10th Circuit rejected his argument in May 1998 and instead ruled that federal law and Congressional policy encourages cooperation between the INS and local police. By declining to hear an appeal of the ruling, the Supreme Court confirmed that whenever state law gives police the authority to enforce federal law, local and state law police officers with "probable cause" can investigate and arrest aliens suspected of federal immigration crimes.

Immigration crimes include illegal entry, illegal presence, smuggling, harboring, or transporting illegal aliens, use of false documents, or making false statements regarding immigration status.

The 10th Circuit held that no federal law preempts the authority of state or local police to enforce federal laws. Under the Constitution, a valid federal law preempts a contradictory state law. The ruling directly contradicts the key argument made by the district judge who blocked California's Proposition 187 in 1995. The judge struck down provisions which required California police and government agencies to question all arrested persons about their immigration status and report suspected illegal aliens to the INS. The judge had claimed that state investigations which had a substantial effect on immigration were preempted. "This ruling strengthens the claims by supporters of the California initiative that Governor Davis sold out the voters who enacted Proposition 187 when he refused to appeal the district judge's ruling," added Stein.

The court declared that the extensive immigration reform and antiterrorist legislation passed by Congress in 1996 was evidence of a federal policy to encourage local police to participate in enforcing the nation's immigration laws.