(CNN) -- Tagg Romney, a son of GOP presidential hopeful Mitt Romney, received personal assurances from the owner of a landscaping company that the firm no longer employed illegal immigrants, a senior Romney aide told CNN.
Mitt Romney sparred with Rudy Giuliani over illegal immigration during last week's CNN/YouTube debate.
The aide said Romney's son believed the owner, himself a legal immigrant, because he was a good friend of the family.
The candidate on Tuesday fired the landscaping company that worked on his Boston, Massachusetts, home when he said he learned it still employs illegal immigrants despite assurances otherwise.
The announcement came after The Boston Globe approached Mitt Romney's campaign with what an official called "credible information" that the company continued to employ illegal workers -- including some at the home of the former Massachusetts governor.
"I am disappointed that our relationship must end on this note," Romney wrote in a letter to the firm that his campaign released.
"But we simply cannot tolerate your inability to ensure that your employees are legally permitted to work in the United States."
With Romney locked in a tight race in the GOP's upcoming Iowa caucuses, the company -- which the candidate said he gave a "second chance" after similar concerns arose last year -- had become an easy target for his political opponents.
"The company's failure to comply with the law is disappointing and inexcusable, and I believe it is important I take this action," Romney said in a statement.
Romney said he met with the owner of Community Lawn Service last year and agreed to keep using it after the landscaper guaranteed him he would only hire legal workers.
Immigration is a major topic in the race for the Republican nomination, with candidates working to position themselves as tough on workers entering the country illegally.
During a CNN/YouTube debate last week, former New York Mayor Rudy Giuliani attacked Romney for living in a "sanctuary mansion" where illegal immigrants were allowed to work. Watch the Giuliani-Romney exchange »
Romney said the company, not him, hired the workers and called it "offensive" to suggest he should have checked the employees' immigration status.
"Are you suggesting ... that if you have a company that you hired to provide a service that you are now responsible for going out and checking the employees of that company, particularly those that might look different or don't have an accent like yours?" Romney said during the debate. "I don't think that's American."
Giuliani's campaign said Tuesday that "the Romney statement speaks for itself" and offered no further comment. E-mail to a friend
CNN's Dana Bash contributed to this report.