-- Police and the mother of a 4-year-old California boy clung to hope Thursday for the child's safety, days after a man allegedly snatched the pre-schooler from his grandmother's arms then took off.

An Amber Alert was issued for Juliani Cardenas, who authorities say was abducted by Jose Esteban Rodriguez in the northern California town of Patterson.

"He (Rodriguez) wanted to be with my son," Tabitha Cardenas told reporters Thursday, adding that she had broken up with Rodriguez four or five months ago. "He wanted to be with my son so bad that he took it to the extreme."

Yet authorities feared the worst, after a witness reported seeing a car matching that of the alleged kidnapper plunge into the Delta-Mendota Canal. But they were not able to locate the vehicle during searches Thursday.

"We believe the suspect drove the car into the canal, with little Juliani inside," Stanislaus County Sheriff Adam Christianson told HLN's Nancy Grace. "But so far, we haven't been able to locate the car."

Christianson said that the search will resume Friday, with help from the neighboring Merced County sheriff's department, using side-scanning sonar. The sheriff, though, also held out hope that the witness may have been mistaken -- and Juliani, in fact, never went underwater.

"There's still hope that we're going to be able to find Juliani alive," he said.

In an emotional press conference Thursday, the boy's mother said she believes Rodriguez is troubled -- claiming he told her that he sometimes "hears voices" and that he had been abusive when they were dating -- but she did not think he would drown her son or consciously put him in danger. Rodriguez is not the boy's biological father.

"He's scared, Jose's scared -- I know he is," said Cardenas. "He probably doesn't know what to do."

The suspect had come to Cardenas' house early Tuesday morning, according to the boy's mother, leaving after she told him to leave because Juliani was asleep and had a cold. The boy's school later called Tabitha Cardenas after Rodriguez had gone there asking to spend time with the boy.

In the late afternoon, Cardenas got a call from her frantic mother, who claimed that the Rodriguez had again come by and, despite her struggle, had taken the boy from the grandmother's arms. Cardenas said that her mother told her that the suspect then flashed an obscene gesture, laughed and got into his 2003 silver Toyota Corolla, which he had left running.

The boy, Juliani, is 3 feet, 6 inches tall, weighs 40 pounds and has brown eyes and black hair, according to the Amber Alert. He was wearing dark pants and a light blue shirt at the time of his abduction.

Rodriguez, 27, is described in the same alert as being a Hispanic male, about 180 pounds and 5 feet, 9 inches tall.

Tabitha Cardenas, who broke down several times while addressing reporters, said she especially was worried that her son was not getting enough water or food. She issued an urgent plea to Rodriguez, who she said took their break-up hard but stayed in contact, to drop off her son at a hospital or fire department.

"Please, I need him home," she said. "I know he's telling you that he's missing me. You need to bring him home."