Secretary of the Interior Ken Salazar has apologized to a Colorado Springs newspaper reporter after ridiculing and physically threatening him during an encounter earlier this month at a campaign event for President Obama.

The exchange, caught on audiotape by reporter Dave Philipps of The Gazette of Colorado Springs and later posted online by the paper, shows Salazar becoming aggressive after questions about the alleged sale of hundreds of federally-protected wild horses for slaughter and whether additional safeguards are needed.

"Do you see any changes that need to take place in that system in terms of safeguarding those horses?" Philipps is heard asking.

Salazar replied calmly, pledging that the agency is "working on it very, very diligently." But then, amid rustling of the recorder when it seems the interview had ended, Salazar delivered a threat.

"You know what, never do that. This is a - this is the Obama - You know what, if you do that to me again, I'm going to punch you out. OK?" Salazar told Philipps in the recording. "Don't ever, ever, from the Gazette or anybody else do that to me again. Set me up. You know?"

Phillips defended himself, saying that Salazar was there "as a public figure" and that previous inquiries through the department's press office had not been answered.

"I'm here for President Obama. I'm talking about the campaign. You want to talk to me about a public policy issue, you can do that on another time, in another place," Salazar said.

Blake Androff, a spokesman for Salazar, told ABC News the secretary "regrets" the exchange and personally apologized to Philipps by phone yesterday. A post on the Gazette's website confirms that an apology was made both by phone and in writing.

"As I said to you by phone today, I apologize and regret the statement I made to you in Fountain, Colo.," Salazar wrote in a letter posted on the site. "I also will be happy to speak with you officially on the BLM Wild Horse and Burro Program."

Philipps, a one-time finalist for the Pulitzer Prize, recently published an investigative report for ProPublica that examined ties between Salazar and a Colorado businessman who allegedly sold federally-protected horses for slaughter. The secretary did not respond to requests for comment before the piece was published, the Gazette said.

You can hear audio of the exchange HERE.

Read Salazar's apology letter HERE.