WASHINGTON (AP) — Ben Carson is defending his continuing business relationship with a close friend convicted of defrauding insurance companies.

At a campaign stop in South Carolina on Friday, the Republican presidential candidate said he has no regrets over helping Alfonso A. Costa avoid prison time. Carson appeared as a character witness at the 2008 sentencing hearing for the Pittsburgh dentist, who pleaded guilty to health care fraud.

The Associated Press reported Thursday that Carson has invested millions in real estate deals through a firm run by Costa, whose dentistry license was revoked following his felony conviction. According to financial disclosure forms, Carson and his wife make between $200,000 and $2 million a year from those investments.

In a 2013 book, Carson called for severe penalties for those convicted of health care fraud.

Copyright 2015 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.