They feared Porter deliberately planned the crash to get his hands on the multimillion-dollar trust fund of Frank Craig, an 85-year-old farmer.
Now authorities believe they have enough evidence to confirm those fears.
Porter, 55, of La Grange, was detained this week at a border checkpoint at San Ysidro while attempting to return to the United States from Mexico, where he was starting a new mission, authorities said.
He was expected to be returned to Stanislaus County on Thursday to face charges of murder, attempted murder and embezzlement from an elderly person.
A well-known high school wrestling coach, Porter was pastor at Hickman Community Church, a nondenominational Christian church, when he befriended Craig.
The elderly man hired Porter to help him develop a museum in Hickman, a town of about 450 residents some 90 miles north of Fresno, said Stanislaus County sheriff's detective Mark Copeland.
Eventually, Porter was named trustee of Craig's reported $4 million in stocks and real estate left to him when his brother died, Deputy Royjindar Singh said. Family members were replaced by the church as heir, Craig's family told authorities.
Porter spent money on museum plans, but nothing was ever built, authorities said.
Investigators said they believe Porter first tried to kill Craig in 2002 when he veered his truck off a rural road and struck an oak tree. The crash crippled Craig, but did not initially raise suspicions.
In April 2004, Porter plunged his pickup truck into an irrigation canal, Singh said. Craig drowned, while Porter walked away from the crash.
"The first crash we had investigated as an accident," Singh said. "But after the second one, the family came to us and said there was something more going on."
Craig's family sued Porter for fraud, contending the elderly man's two sisters were deprived when the trust was changed to benefit Porter's church, said attorney David Jamieson. The family and Porter also settled a wrongful death lawsuit, Jamieson said.
The criminal case languished as prosecutors focused on convicting Scott Peterson and sending him to death row for the murder of his pregnant wife, Laci. Porter resigned as pastor last year as suspicions about him mounted.
His home was recently listed for sale at $895,000, but has been taken off the market, real estate agents said.
The district attorney's office plans to file charges when Porter is returned from San Diego, where he's being held without bail, said Deputy District Attorney John Goold. Porter is expected to be arraigned Friday.
"We're just at the start of this and have to wait and see how the case plays itself out," Goold said. "He is still innocent until proven guilty."