Police said a regular customer was also inside the Pride Truck Stop on South Anderson Road when it happened. He ran to the Waffle House next door and called 911 as well. In fact, there were multiple calls shortly after the attempted robbery.
Detectives believe only one shot was fired during a struggle between the clerk and the robber.
Store employees said the 22-year-old had only worked nights at the busy truck stop, located near Interstate 77, for a few weeks. One woman said he worked the late shift so female employees wouldn't have to out of concern for their safety.
Late Monday, Rock Hill police released surveillance video of the incident after struggling for most of the day to overcome technical issues that kept them from watching it.
Those problems, along with the lack of a good description of the suspect, made for a frustrating start to a murder investigation.
The video shows a black man wearing a heavy, dark-green hooded jacket and dark pants walk into the store just before midnight Sunday. He goes straight to the counter and points a handgun at Salem. Moments later you see Salem grab a knife from near the register and walk out of the camera's view, hiding it behind his back.
Police said there's more to the video, but they don't want to release it. The two men struggled before the clerk was shot, and the gunman may have been stabbed.
"He may have a cut on him somewhere," said Lieutenant Brad Redfearn. "Please call us if you see anything like that."
The Pride station is always open, and regular customers described the young clerk as kind and helpful.
"He was [a] super nice young guy, a big guy, but he was always treating people friendly every time you came in here," Ervin Ratterree said.
"I stop here every morning. Today I saw all the lights off and thought something was going on, but I didn't know," Stacy Blount said.
Police detectives went back to the store late Monday to walk through a nearby grass field, hoping to find any evidence that the suspect ran that way.
Right now, they have no idea which way he ran, or if he jumped into a waiting car. The surveillance video of the parking lot apparently shows nothing.
Family members who help run the store didn't want to speak on camera. Salem also has close family in Yemen and Rochester, N.Y.