Police in northern Mexico on Tuesday discovered a Ford Mustang (search) belonging to a man sought in the disappearance of a 16-month-old boy whose mother was found dead.

Ivan Villa (search) was believed to have been driving the car found in Ciudad Juarez (search) and later secured at a Mexican police station. No other details were released.

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

Authorities did receive a tip Tuesday afternoon that a man carrying a baby was seen walking across one of the international bridges from Juarez to the United States. Glascock said investigators were trying to determine if the information was relevant to the case.

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, who is not Villa's, were gone. Police issued a warrant for Villa's arrest on a charge of kidnapping.

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

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

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."