(Associated Press) Democratic presidential contender Michael Bloomberg has cut ties with a contractor that used prisoners to make calls for his presidential campaign.

The online news site The Intercept reported Tuesday that Bloomberg’s campaign contracted a New Jersey-based call center company that, in at least one instance, used Oklahoma inmates to make calls on behalf of the billionaire’s campaign.

The former New York mayor says he was unaware of the arrangement until it a reporter raised it. Bloomberg says in a statement that the campaign is making sure its vendors more properly vet subcontractors in the future.

Photo Credit NYC.gov, showing Michael Bloomberg announcing the budget for fiscal year 2015