(CNN) -- Six of the seven college students killed in a massive house fire Sunday morning in Ocean Isle Beach, North Carolina, attended the University of South Carolina, a university official said Sunday.
"This is a very sad day for the University of South Carolina family," Dennis A. Pruitt, the vice president for student affairs, told reporters.
The seventh fatality is believed to be a Clemson University student, he said.
The six survivors -- who were treated and released from Brunswick Community Hospital -- are also USC students, he said.
Pruitt said the bodies were being sent to Chapel Hill, North Carolina, for identification. Watch home video showing the house in flames »
"It is difficult to ascertain exactly who was in the house and who was not," Pruitt said, explaining that several area houses were filled with college students visiting for the weekend.
Newspaper deliveryman Tim Burns was aghast when he first saw the inferno early Sunday morning. He tried to approach the door but the flames were too intense, he told the Associated Press.
"When I was going up to the entryway, you could hear the windows above me explode," Burns said. "When I knew the flames had taken over, I don't think I've ever felt as helpless in my life."
The USC students were affiliated with the Delta Delta Delta sorority and the Sigma Alpha Epsilon fraternity, Pruitt said, although he stressed that the weekend was not an official Greek function.
"It just so happens, as on many university campuses, that certain sororities and certain fraternities hang around together," he said.
One person described as "unaccounted for" was later confirmed dead, Ocean Isle Beach Mayor Debbie Smith told CNN.
"We suffered a terrible tragedy at Ocean Isle this morning," Smith said in a news conference. "Nothing like this has ever happened at Ocean Isle Beach, and we hope it never will again."
She said officials had not yet contacted all the victims' families, and that they were not yet releasing their names. The mayor said one of the 13 people in the house was related to the house's owner.
Smith said the house was fully "engulfed" in flames around 7 a.m. when the fire department arrived on the scene, about five minutes after the fire department was notified.
She said the house had working smoke detectors, AP reported.
Officials are investigating the cause of the fire, Smith said.
"We ran down the street to get away," Nick Cain told the AP. The University of North Carolina student was staying at a house about 100 feet away.
Some of his friends had met several people staying at the house that burned down.
"The ash and the smoke were coming down on us. We were just trying to get away," Cain told the AP.
Ocean Isle Beach is a popular resort destination along the southern coast of North Carolina. The 7-mile-long island's year-round population is about 425, while the summer season population is about 25,000, according to the town's Web site. E-mail to a friend