The plane's pilot was also killed in the crash. Family members identified him as 35-year-old Jason McLaughlin of Weston, Florida.
The plane had left the Dominican Republic late Monday when it crashed in the coastal town of Arecibo as it headed toward Puerto Rico's main international airport. The U.S. Federal Aviation Administration said air traffic controllers lost contact with the flight about 25 miles (40 kilometers) west of the capital of San Juan.
No details of the investigation have been released. The FAA identified the aircraft as a Fairchild SA-227-AC cargo plane.
Greg Gullberg, who began flying with his brother when they were teenagers, said Steve Gullberg lived in Puerto Rico and knew the route well.
"They had flown it numerous times," he said. "They knew what they were doing."
He said his older brother attended space camp as a boy and his childhood room was painted with airplanes and cockpits.
"Steve has wanted to be a pilot since forever. He always wanted to follow my dad's footsteps," he said of their father, a retired American Airlines pilot.
A former IBC Airways pilot who spoke on condition of anonymity for fear of reprisals from the airline industry said he also worried about the safety of the planes when he worked for the company.
"The planes are very old," he said. "They're always having mechanical problems, having to be fixed."
The FAA in October 1986 awarded the plane that crashed a certificate of airworthiness that was valid through 2017.