The mystery over what happened to a 16-month-old boy, believed taken after his mother was strangled, deepened after authorities in Mexico found the car the kidnapping suspect was driving.

Police in northern Mexico on Tuesday discovered a Ford Mustang that they believe was driven by Ivan Villa (search). The car was found in Ciudad Juarez (search) and later secured at a Mexican police station.

"We're still investigating to determine whether he is inside or outside the country," New Mexico state police Lt. Jimmy Glascock said.

Authorities were investigating a tip that a man carrying a baby was seen walking across one of the bridges from Juarez to the United States. They were also looking for a white Cadillac Escalade with Texas plates that was spotted trying to tow away the Mustang.

Glascock said police haven't received any information indicating that Villa would hurt the child, but he added that police were proceeding with caution. Villa is not the child's biological father.

"Certainly we're focused on getting the child back to his family safe," Glascock said.

Police on Sunday had issued an Amber Alert for the toddler, Justin Black (search), resulting in a nationwide search.

The boy was at home in a Ruidoso, N.M., apartment when police responded to a call of a verbal dispute Sunday afternoon between Ivan Villa, 22, and Kristi Black, 19.

Several hours later, a friend went to the apartment and found Kristi Black's body. The state Office of the Medical Investigator determined she was strangled.

Villa and the child were gone. Police issued a warrant for Villa's arrest on a charge of kidnapping.

Kristi Black's mother, Dee Dee Black of Chowchilla, Calif., waited anxiously for word that her grandson was safe. She urged Villa to contact her.

"I was one of the few people he could talk to. I'd like for him to call me now," she said. "I know he's probably very confused. But, I'd like to tell him to bring Justin back to us."

Dee Dee Black said she kept in touch by phone with Villa and her daughter since they married last year. He appeared to be a caring husband and strong father figure to the toddler, she said.

While police haven't implicated anyone in Kristi Black's death, Dee Dee Black said she's not convinced that Villa played a role.

Dee Dee Black said she last spoke to the couple a week before Mother's Day.

Dee Dee Black said her daughter was a devoted mother who sometimes e-mailed her photos of her son. She became emotional recalling their last conversation.

"It hurts. You don't expect your daughter to go before you, especially in a tragic and unsettled murder," she said. "And now, Justin is missing, too."

The Associated Press contributed to this report.