GREELEY, Colo. -- Opening arguments and early testimony got under way Monday in a lawsuit against Weld County authorities over tax records seized in an identity theft investigation.

The American Civil Liberties Union sued District Attorney Ken Buck and the Weld County Sheriff's Department over their investigation targeting more than 1,300 illegal immigrants.

Buck said the immigrants were filing federal taxes using false or stolen identities. The ACLU said the tax records are confidential and authorities had no right to seize them from Amalia's Translation and Tax Services in Greeley.

Amalia Cerillo, owner of the tax service business, took the stand Monday afternoon and told the court her business has been greatly impacted by last fall's raid, when investigators removed 5,000 hard copy files from her office.

She said her clients don't trust her now and she's losing money.

Attorneys for the Weld County Sheriff's Office tried to get her to admit that she knew some of her clients were in the country illegally. She cried, and said it was her job to file her clients' taxes, regardless of their status.

The ACLU is demanding that authorities return or destroy the tax files taken from the business.

Prosecutors say 54 identity theft cases are pending, in addition to six that were filed through a grand jury. Some defendants have already pleaded guilty to charges of identity theft and criminal impersonation.

District attorney's spokeswoman Jennifer Finch said those who have plead guilty can receive a four-year suspended prison sentence if they comply with deportation proceedings.

Immigrant advocates, like the Colorado Immigrant Rights Coalition, say the people charged are being punished for doing what the law requires them to do -- pay taxes.

Regardless of legal status, people who earn income in the U.S. are required to pay taxes.

Buck said the defendants were breaking the law by being in the country illegally in the first place.

The case is scheduled to last through Tuesday. The judge could rule on the case at the end of the hearing or at a later date.