They also were withholding ownership of the plane.
Federal Aviation Administration (FAA) records indicated Sunday that the plane was registered to Diamond Flying LLC, a St. George business.
Video of the runway showed the plane crashed after takeoff at about 1:20 a.m., said Zoe Keliher, investigator for the National Transportation Safety Board.
"The airplane continued down the runway and made a rapid ascent," Keliher said. The plane left the frame of the video and then reappeared, descending rapidly, Keliher said.
The wreck was found about 5:30 a.m. during a routine security check, Keliher said. All four men had died.
Brecken Chapman, Chapman's 13-year-old sister, said her brother cared about others and always put them ahead of himself. She said he planned to attend the University of Utah this fall in hopes of becoming a doctor.
"He was really funny," Brecken Chapman said from the family's home. "He had a memorable smile. He was a good brother and a hard worker."
She said she saw her brother the night before the crash but didn't know he planned to go flying with friends. She said her brother hadn't flown frequently.
Investigators had not yet confirmed the destination or reason for the flight, Keliher said. The cause of the crash remains under investigation.
Mortensen said employees perform the last check at the airport at midnight and return at 6 a.m. The airport does not have a radio tower. An automated system allows pilots in the area to communicate with one another during takeoff or landing.
Investigators moved the wreckage Sunday to a hangar at the airport so it could be further analyzed over coming weeks, Keliher said. The FAA and the National Transportation Safety Board are investigating the crash.
The airport does not plan to review its policies, Mortensen said. A police officer performs perimeter checks at night, he said, noting the city may increase those checks. However, he noted the outcome would have been the same even if employees had been there.
"If he would have been on the scene moments later, it would not have made a difference," Mortensen said. "It was obvious those involved were killed on impact."
Keliher said a preliminary report will be complete in about five days, with the board's final conclusions to be issued in six to eight months.
Investigators are trying to locate any witnesses who may have seen the crash or had contact with the men beforehand. Keliher encouraged anyone with information to contact her at firstname.lastname@example.org.