The ACLU has asked the Colorado Supreme Court to uphold a District Court ruling that stopped the investigation in April. The judge ruled that Weld County authorities violated people's privacy and had no probable cause to inspect so many confidential taxpayer records.
Weld's sheriff and district attorney said in their appeal last month they had substantial evidence to believe hundreds of suspected undocumented immigrants were stealing people's identities to work in the U.S.
The Supreme Court has not set a date to hear arguments in the case.
Authorities filed identity theft and criminal impersonation charges against more than 70 people before the investigation, dubbed "Operation Numbers Game," was stopped.
The investigation began in October after a Texas man told Greeley authorities someone was using his identity. The suspect in that case told authorities he was filing his taxes with a Greeley tax preparer that catered to Latinos in the city about 60 miles north of Denver.
That prompted the sheriff and district attorney to search the business of tax preparer Amalia Cerrillo, hoping to find proof that people were working with Social Security numbers that weren't theirs, and filing taxes with government-issued taxpayer identification numbers.
Those numbers are typically issued to people who don't have Social Security numbers to pay taxes, which is required of everyone who earns wages in the U.S. regardless of legal status.
About 10.7 million taxpayer numbers were assigned between 1996 to 2007, and people who used them have a tax liability in the billions of dollars, the Internal Revenue Service has said.
The ACLU said in its Supreme Court brief that except for the suspect accused of stealing the Texas' man's identity, investigators had no probable cause to search the thousands of records of other taxpayers.
The ACLU said the investigation of Cerrillo's business was akin to getting a search warrant for a hotel where drug dealers are known to stay, and then going through every room to look for drugs.
Four district judges have agreed with the ACLU's argument that Weld County's search warrant was unconstitutional; one judge called it "breathtaking in its expansiveness."
Weld County has said it was impossible to identify individual suspects in the search warrant because the case centered on identity theft.
Immigration experts have said this is the first and only time authorities have used confidential records from an income tax preparer to prosecute undocumented immigrants.
The Mexican American Legal Defense & Education Fund also filed a brief with the Supreme Court Friday supporting the ACLU's arguments. That brief says the confidentiality of tax records is necessary to avoid "the creation of an underground economy," in which illegal immigrants don't comply with tax laws, depriving the government of much-needed revenue.
(Copyright Associated Press, All Rights Reserved)