By Joel Schectman

WASHINGTON (Reuters) - U.S. authorities said insurance providers Humana Inc and AXA Equitable Life Insurance Co violated sanctions by providing policies to three people blacklisted for alleged ties to narcotraffickers.

The U.S. Treasury Department Office of Foreign Assets Control (OFAC), which enforces sanctions law, issued findings of violations to the two providers Tuesday but did not issue fines.

While the companies will not face a monetary penalty, the violation notices show how strictly U.S. authorities can enforce the letter of the law in sanctions compliance.

The companies had been servicing the three customers Leopoldo Lopez Grayeb, Noemi Lopez Fernandez, and Juan Manual Lopez Fernandez long before they were added to the U.S. sanctions blacklist in 2009 for alleged ties to Mexican drug cartels, according to the notices.

In 2012, OFAC removed the three from the blacklist, without further explanation, according to records on the agency's website.

But during the three years when the three were designated, the insurers failed to check the customers against OFAC's blacklist, OFAC said. Spokespeople for the companies did not respond immediately to requests for comment.

Sanctions make it a crime to do any business with those on the blacklist, even if there is a pre-existing relationship, and regardless of whether U.S. officials later change the policy, said Adam M. Smith, a sanctions attorney at the law firm Gibson, Dunn & Crutcher LLP.

Even though the transactions were not seen as bad enough to warrant a penalty "you still don't get out from having a violation," Smith said.

Erich Ferrari of Ferrari & Associates PC said the violations puts the insurance industry on notice "even though the fact patterns are not routine, these are the kind of activities you need to watch out for."

(Reporting by Joel Schectman; Editing by Bernard Orr)