The father of a promising, young American explorer and National Geographic photographer who was struck in the face by a melon-sized chunk of cement nearly a week ago in South Africa says his son "is struggling" to live, and that "the next 36 hours are going to be critical."

Bob Model told FOXNews.com that his son, 34-year-old Bobby Model, remained unconscious, but was not in a coma.

"He was off life support, but they put him back on tonight," Model said.

Cape Town police, meanwhile, continued their investigation into what caused the cinderblock to come crashing down from an overpass and through the windshield of a pickup truck last Thursday night, hitting the adventurer in the face.

Model, a Wyoming native, was in South Africa for a visit with his 32-year-old sister Faith, who was driving the vehicle.

Bob Model said there was a heavy rainstorm the night the car was struck.

"It’s still under investigation," he said. "Who knows what happened. Whether it was targeted toward him — I don't think so. But we just don't know that."

It wasn't immediately clear whether someone hurled the block of concrete at the vehicle, or whether it accidentally broke loose and fell off a truck — though incidents of people throwing rocks at highway traffic are common in Cape Town. Authorities were treating it as an attempted murder case.

Model's sister escaped unscathed, but her brother suffered severe head and brain injuries and slipped into a coma. Over the weekend, he was listed in critical condition with his sister at his bedside, according to media reports. By Tuesday, Model had stabilized, though he was still unconscious and in critical condition in the intensive care unit, a spokeswoman for the hospital, Vergelegen Medi-Clinic, told The Independent Online.

Model, who has been based in Kenya for work, was driving with his sister near the Monwabisi resort, close to the impoverished Cape Town township of Khayelitsha, when the tragedy happened.

The National Geographic Society named Model one of its "emerging explorers" last year, an honor reserved for "dynamic personalities [who] are already making significant contributions to their fields and show potential for true breakthroughs," according to Nationalgeographic.com.

The photographer — who graduated from the University of Wyoming in 1997 with an economics major and has been rock-climbing since he was in his teens — is a contributor for National Geographic Adventure magazine. He appeared on the cover of National Geographic after he and other members of a Wyoming climbing expedition made the first free ascent of the East Face of Trango Tower in Pakistan.

"He’s a young, great American explorer," Bob Model said of his son. "He has made a wonderful career as an adventure photographer. We’re very proud of him — we think so much of him.

"We think it’s important that his story is told."

The Associated Press contributed to this report.