A federal jury has acquitted US poultry processor Tyson and three Tyson managers of conspiring to hire illegal immigrants from Mexico and Central America to work at the company's poultry plants.

The jury deliberated for less than a day before acquitting the defendants of all charges in a case that followed a four-year investigation during which undercover agents posed as immigrant smugglers and secretly recorded conversations with managers of Tyson poultry plants, reported the Associated Press.

"We are extremely gratified by the jury's decision. The verdict confirms that Tyson Foods has made a concerted effort to hire properly and abide by the law. We will continue our efforts to make sure each person we hire has proper documentation," Tyson Foods spokesman Gary Mickelson said after the verdict.

US Attorney Harry Mattice Jr. said the verdict as "frustrating. ... We are consoled by the fact that we did the right thing by bringing this prosecution."

Six Tyson managers were charged along with Tyson in December 2001. One manager fatally shot himself a few months later, while two others made plea bargains in January and testified for the government.

The three individual Tyson officials on trial – regional vice president Robert Hash, plant manager Keith Snyder and retired division human resources manager Gerald Lankford – testified that they were unaware that illegal immigrants were being hired.

Tyson attorneys claimed that the company and its managers were victims of the government’s imperfect system of tracking immigrants.

Following the verdict, jurors said the case for the prosecution had been unconvincing.

"We felt like the government didn't properly present its case. There were a lot of loopholes," Juror Debra Goldston was quoted by the Associated Press as saying.

Fellow juror Barbara Hailey said prosecutors failed to prove a conspiracy at Tyson that went beyond the managers who pleaded guilty.

"Nobody ever mentioned (the defendants') names. They were never recorded, they were not on any tapes," she said.

If the defendants had been found guilty, the three managers would have faced fines and prison sentences, while Tyson would have faced millions of dollars of fines.