PORTLAND — The owners of three popular Mexican restaurants in Maine who were convicted of harboring undocumented workers have been granted a new trial because one of the jurors in their first trial used a racial slur to refer to them.

Guillermo Fuentes, 37, of Westbrook, and Hector Fuentes, 39, of Waterville, were found guilty March 18 of conspiracy, harboring undocumented aliens for profit, and aiding and abetting document fraud.

Judge D. Brock Hornby granted them a new trial, tentatively set for Oct. 7, after learning that the juror made the slur to a man at the Eagles Club in Portland on March 9, the second of seven days of trial testimony, according to documents made available this week in U.S. District Court.

The man at the club was on probation at the time and reported the conversation to his probation officer. He reported that the juror said he wasn’t supposed to be talking about the case, but the defendants were guilty anyway, and he referred to them with a racial slur, according to the judge’s order.

The probation officer did not learn the juror’s name until April.

The judge said in his 23-page order that he interviewed the man who was on probation and the juror, one in May and the other in June. He said the juror first denied making the slur, but ultimately admitted it.

Hornby said in his order that the juror’s comment raised two important concerns: “that at that early stage of the trial this juror had already made up his mind that the defendants were guilty, and that ethnic stereotyping affected his judgment.”

The U.S. Attorney’s Office says that, from 2006 to 2011, the brothers helped undocumented workers at the Fajita Grill in Westbrook, the Cancun Mexican Restaurant in Waterville and the Cancun Mexican Restaurant II in Biddeford get green cards and Social Security cards.

The Fajita Grill, known for its fast service, lively staff and supersized margaritas, became a hot spot in Westbrook soon after it opened.

Several workers testified during the eight-day trial that they worked six or seven days a week, from 9 a.m. to 10 p.m., with only one two-hour break. Kitchen workers said they were paid in cash. Waiters were not paid but were allowed to keep their tips.

The brothers face 10 years in prison and a $250,000 fine on each count, and they may have to give up their profits if they are convicted again.

The Department of Homeland Security’s investigation into employment practices at the Fajita Grill was prompted by a tip from Westbrook police Capt. Tom Roth, court documents say.

The documents say Roth told the federal agency that in routine traffic stops in April 2008, Westbrook officers pulled over Hispanic men who appeared to work at the Fajita Grill, claimed to be from Mexico and could not provide any U.S. identification.

James Bell, a special agent with the Department of Homeland Security, interviewed four workers, all of whom had worked for the Fuentes brothers at a Mexican restaurant in Atlanta called El Potrillo.

They said they moved in 2006 with about a dozen other employees to work for the brothers at the Fajita Grill and later at one or both of the Cancun restaurants.

They told Bell they were paid $300 to $500 per week in cash for working about 60 hours.

Three of the people who were interviewed reportedly told Bell that when they arrived in Maine, they and seven or eight other undocumented workers temporarily lived rent-free in the basement of the Fajita Grill.