Updated

Immigrant advocates say they've seen nothing like it before or since: A prosecutor looking for illegal immigrants seized thousands of confidential tax records from an income tax preparer popular with Hispanics in this northern Colorado city.

The October seizures led to identity theft and criminal impersonation charges against more than 70 people, and prosecutors allege that as many as 1,300 suspected illegal immigrants were working using false or stolen Social Security numbers.

But the American Civil Liberties Union said the documents of as many as 4,900 people were seized, many of them legal residents, and that the probe was the "equivalent of a house-by-house search of innocent homeowners in order to find a suspect believed to be somewhere in the neighborhood."

Two judges have agreed, ruling that Weld County District Attorney Ken Buck had no probable cause to seize the records. Buck is appealing, however, and a ruling in his favor could open up a new avenue for prosecuting illegal immigrants.

The charges have been ironic for immigrants like Horacio Arturo Cervantes. The 42-year-old father of four from the Mexican state of Chihuahua said he had been honest about paying taxes, even though he was in the country illegally, because he was hoping for a path to U.S. citizenship.

Cervantes pleaded guilty to identity theft before the judges' rulings, and now faces deportation. He said he pleaded because he wanted to get out of jail and try his chances in immigration court.

"I feel like I'm up in the air, not knowing what's going on, just with a desire to stay here," he said in an interview in Spanish. His next court date is June 23.

Buck's investigation, dubbed Operation Numbers Game, marked the first time a prosecutor used tax records to charge illegal immigrants with identity theft, according to the ACLU and the Los Angeles-based National Immigration Law Center. Officials with both groups said they knew of no prosecutor who has tried it since.

Buck's probe triggered a conundrum: The people charged allegedly are in the country illegally and were fraudulently using Social Security numbers. But the Internal Revenue Service requires them to pay taxes, and those records are confidential.

Buck declined to be interviewed for this article because of pending appeals. But in a February interview, he argued that "if you're in this country illegally, and you're working in this country illegally, there may be a requirement that you pay taxes but it's kind of ridiculous."

Buck has argued that the records aren't confidential because they were seized not from the IRS but from the tax preparer.

The investigation started after a Texas man alerted Weld County authorities that his identity was being used. The suspect in that case told authorities he had filed his taxes with Amalia's Translation and Tax Services, a business widely used by immigrants in Greeley, where one-third of the population is Hispanic.

To pay their taxes, Amalia Cerrillo's customers used Individual Taxpayer Identification Numbers, which the IRS issues to people without Social Security numbers. Foreign nationals with U.S. investment income also use ITINs. Those charged in Operation Numbers Game allegedly used others' Social Security numbers to work and their own ITINs to pay taxes.

ITIN filers had an income tax liability of $50 billion from 1996 to 2003, according to the latest IRS figures available. Former IRS Commissioner Mark Everson told Congress in 2006 that ITINs were "bringing taxpayers into the system."

IRS spokeswoman Nancy Mathis said ITINs solely track tax returns and don't convey or record immigration or work status. Nor do they give immigrants the right to work in the U.S.

Buck had said more identity theft cases were possible, and that prosecutors in other states were interested in his investigation, but Larimer County District Judge James Hiatt halted it in April. Ruling in favor of the ACLU, he said investigators violated privacy rights by seizing the tax documents and that their search was over-broad. Hiatt likened the search to taking medical records from a doctor's office because one patient was a suspected drug user.

In a separate but similar case in March, Weld County District Judge James Hartmann Jr., called the search warrant "nothing more than an exploratory search based upon suspicion that some unknown person or persons" committed a crime.

Buck is appealing the rulings, and one of the appeals is before the state Supreme Court. While the cases he had pending were allowed to proceed, he has dismissed 30 of them without prejudice so far. The move gives him the option to file charges against the same people again if his appeals succeed.

However, a Supreme Court ruling in early May could be another setback to Buck's investigation. The unanimous ruling said undocumented workers can be considered identity thieves only if authorities prove they knew they were using someone else's Social Security number.

Jeff Joseph, a Denver immigration attorney with four clients who were arrested in Operation Numbers Game, said there are people who will have a difficult time staying in the U.S. even if criminal charges are thrown out.

That's because immigration court imposes a tougher standard than criminal courts. Defendants have to prove not only that the search and seizure of tax records was illegal, but also that the conduct of those executing the search was "outrageous and egregious," Joseph said.

"So, the real tragedy is although the criminal cases will probably be kicked out, there are severe collateral consequences to all of these individuals who were simply working, supporting their families and trying to follow the law by paying their taxes," he said.

Many of those caught in Operation Numbers Game continue to insist they did nothing wrong.

"If it's a crime to pay taxes, then the IRS should also be charged because they're the ones who gave us the ITIN," said Maria Eugenia Rey, 37, who is in the country illegally, along with her husband, Luis. He has yet to enter a plea, and his wife said he has not had charges read to him by a judge.

"He hasn't declared himself guilty, and he won't declare himself guilty because he hasn't committed a crime," said Rey.