(CNN) -- A car pulled from a California canal Friday night was not the same vehicle a witness claimed he saw being driven into the water with a young boy trapped inside, the Stanislaus Sheriff's Department said Saturday.

Authorities are continuing to search the canal near the area where a 4-year-old boy was abducted earlier this week.

Police say Jose Esteban Rodriguez snatched Juliani Cardenas from his grandmother's arms Tuesday afternoon in the northern California town of Patterson.

Divers using sonar equipment were preparing to search the water Saturday, including one section of the 17-feet-deep canal that goes underground, said Raj Singh, a spokesman for the sheriff's department.

"Our hope is that the victim is out there somewhere," Singh said. "It's a good sign that we haven't found anything here. That's why we're encouraging people to keep an eye out and call us with information."

A day after Juliani's alleged abduction, a witness -- described by Stanislaus County Sheriff Adam Christianson as a farm worker in his 50s who was "very familiar with that area" -- saw a vehicle that looked like Rodriguez's 2003 silver Toyota Corolla plunging into the canal with a man and youngster inside.

"We're certain that we're going to find a car," Christianson said Friday. "All the other tips and leads have been unfounded, (and) all of the evidence leads us here. The tips and leads aren't leading us in any other direction."

The boy's mother, Tabitha Cardenas, said Friday that she still believed that Rodriguez -- whom she broke up with about five months ago, and who is the father of her unborn child -- wouldn't purposefully drive her son into the water and that both were still alive.

"My hopes are high that they're not going to find Jose's car in the canal," she told HLN's Jean Casarez.

The car pulled Friday was the third discovered by Stanislaus County Sheriff's Department deputies -- working in conjunction with a dive team from nearby Merced County -- since they began searching the canal this week. The car was determined to be one that was stolen months ago.

Earlier in the day, they pulled out another vehicle that they determined to be a Ford Mustang. The find on Friday night was about a mile away, in a different part of the canal.

Christianson said there's been "no electronic signature of any kind" from Rodriguez since the boy's abduction, meaning no evidence of cell phone, credit card or debit card use.

The sheriff also told reporters that, despite getting "hundreds" of tips, "there are no other credible leads that are taking us in a different direction."

Cardenas said Friday that Rodriguez either wanted to have her son for himself or wanted "revenge" against her, for their break-up. She said that she ended their relationship because he was physically abusive to her, but said that she never thought he'd harm Juliani.

"He (Rodriguez) wanted to be with my son," Cardenas told reporters. "He wanted to be with my son so bad that he took it to the extreme."

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 Toyota, 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.

Earlier Friday, Christianson admitted that the fact new leads have slowed to a trickle and probes into existing ones have largely fallen flat are starting to take their toll.

"This is getting frustrating," he said. "It's like riding a roller coaster: You think you're going to be able to bring closure to the case, then suddenly ... once again, we've recovered probably another stolen vehicle."

Still, a hopeful Cardenas pleaded late Friday for Rodriguez to take care of her son and give him the chance to get home.

"Jose, you're all out of money, come on," she said. "You need to take Juliani to a hospital, leave him at a gas station, I don't care where you leave him. Leave him somewhere ... and just take off."

"I don't care where (Rodriguez) goes, I just want my son back."