From the NYT;  -  “An off-duty officer working for the nightclub responded and engaged in a gun battle with the gunman near one of the entrances.”

http://www.nytimes.com/interactive/2016/06/12/us/what-happened-at-the-orlando-nightclub-shooting.html

From the WSJ;  -  “The shooter opens fire outside Pulse nightclub, engages with an offduty uniformed police officer who was working for the nightclub when the shooting started, and then enters club.”

http://graphics.wsj.com/orlando-shooting/