A test drive of a pickup truck turned deadly for a Richardson salesman.

Now the man he thought was a customer has been arrested for capital murder.

Richardson police still don’t know exactly how this happened, especially as John Phinney, 53, was an experienced salesman. Colleagues say he was big enough and strong enough to handle most situations.

Phinney fell from a moving truck during a test drive on South Central Expressway.

His body was found about 6 p.m. Tuesday near the Spring Valley exit, police said.

James Thorpe, 42, of Dallas was arrested in Lewisville late Tuesday. He was being held in lieu of $3 million bond at a Dallas County jail on a capital murder charge.

Witnesses reported seeing Mr. Phinney fall from the passenger side of the pickup Mr. Thorpe was test driving at about 5:30 p.m. Tuesday, Richardson police said. It was unclear whether Mr. Phinney was pushed from the truck, which was probably traveling more than 50 mph.

A woman had dropped off Mr. Thorpe at Manuel Dodge in Richardson about 5 p.m., said Aaron Miller, a sales manager at the dealership. Mr. Phinney assisted the customer and made a copy of Mr. Thorpe’s driver’s license, which had expired by about 10 years. The two then headed out in a black 2006 Dodge Ram pickup.

Mr. Miller said nothing appeared unusual and that if employees are uncomfortable with a customer, they can deny that person a test drive and ask them to leave.

Mr. Miller said he was surprised by what had happened to Mr. Phinney, who had worked at the dealership for about four or five years. “He never had any ill will towards anybody.” He said. “He was the nicest guy in the world.”

He also said Mr. Phinney could take care of himself. “Jack’s big enough to grab a steering wheel, to grab it if anything happened,” Mr. Miller said.

Employees started realizing something was amiss when Mr. Phinney did not return from the test drive.

Police came by later that night, and the dealership provided a copy of Mr. Thorpe’s driver’s license. Two managers were also brought down to the police station to assist with the investigation, Mr. Miller said.

Police still don’t know exactly what happened, as Thorpe has not been cooperative.

“It appears to be a random thing, where he went into a dealership, and under the guise of wanting to take a test drive, took this truck out on the test drive and Mr. Phinney appeared to be the salesman that assisted him on this,” said Sgt. Kevin Perlich, from Richardson police.

“It doesn’t look as if there