A missing Kansas man spent his final days trapped in the wreckage of his van in a rural Utah ravine, writing goodbye letters to the family he abruptly left in early September.

David Welch, 54, was found Oct. 18 by a hitchhiker who spotted the crash in a desolate stretch of eastern Utah, Utah Highway Patrol trooper Gary Riches said.

Police believe Welch, a retired salesman for Frito-Lay, fell asleep at the wheel of his 2000 Pontiac Montana on Interstate 70 about Sept. 3, a day after he disappeared from his home in Manhattan, Kan. His minivan sped off the road and went airborne, smashing into the side of the ravine. It came to rest upside down on the passenger side, Riches said.

Evidence suggests Welch was injured and unable to get out of the van, Riches said. The medical examiner has not yet determined his cause of death. Even if he had been able to get out, the nearest city, Green River, was about 50 miles west.

Police believe he survived for days, maybe weeks, keeping a journal and writing notes to his wife and four sons.

Thousands of cars sped by on the nearby interstate without a clue — Welch's van couldn't be seen from the highway, Riches said.

"It is very desolate," he added.

Utah Highway Patrol Lt. Scott Robertson declined to describe what was written in the notes, calling them “a pretty private matter,” the Salt Lake Tribune reported.

But Welch’s wife, Kelly, provided further details on Twitter.

“Dave was entrapped in the vehicle down the 50-foot ravine, he wrote each of us a love note,” she tweeted the day police found Welch, the Tribune reported. “He knew he was dying and there was no way out.”

The discovery brought a tragic end to a difficult several weeks for Welch's family, who reported him missing from his home in Manhattan, Kan. on Sep. 2.The family said Welch left in a minivan without telling anyone where he was going, said Riley County Police spokesman Matt Droge.

Welch was put in the national missing persons database.

As the days went on, the family posted missing signs around the city and started a Facebook page to bring attention to the search, Droge said.

On the Facebook page, "Find Dave Welch," they asked people to drive two extra blocks each day in hopes of finding him somewhere in Manhattan. On. Oct. 17, the day before he was found, there was a post written directly to Welch, perhaps in hopes he might read it.

"Dear Dave, it has been 7 weeks since you left. Your wife, children and grandchildren miss you more than the sky is high. Your classmates and friends are concerned for your health and want to help. As we sit at home tonight with tears welling up; Our hearts aching, we wonder where you are. We only pray that you see this message and ask God to bring you home soon. We love you!"

The family declined comment to FoxNews.com on Tuesday evening.

Authorities said they don’t yet know why Welch suddenly left his home without saying anything or why he may have been driving through Utah.

The Associated Press contributed to this report.