(CNN) -- Investigators searching for a missing 8-month-old boy began an intensive excavation and search effort Tuesday at a Texas landfill.



"Let me say this, that we do remain hopeful that baby Gabriel is alive," William McManus, chief of the San Antonio Police Department, said at a news conference at the landfill.



"We are, however, conducting both a missing persons investigation as well as a homicide investigation," he said, adding that aspects surrounding Gabriel Johnson's disappearance involve elements of a possible homicide.



Gabriel has been missing since December 26 and was last seen in San Antonio, with his 23-year-old mother, Elizabeth Johnson, who has refused to disclose information on his whereabouts.



She told Gabriel's father she killed the boy and has also said she gave him away to a couple in San Antonio, police say. Johnson drove Gabriel to San Antonio from Tempe, Arizona, and she went to Florida a week later, according to investigators.



Johnson was arrested in Florida and extradited to Maricopa County, Arizona, where she remains behind bars, charged with kidnapping, custodial interference and child abuse.



A lead prompted investigators to focus on the landfill, which is owned by Republic Services, a waste and environmental services company based in Phoenix, Arizona. The part of the landfill that will be searched has been pinpointed, according to McManus.



First, it will take at least six days to remove 45 feet of garbage that has piled up since the time investigators believe evidence may have been dumped in the area.



"Once we have removed the debris and have searched our target area, we will begin the arduous task of sifting through layers in the search for possible evidence," McManus said.



"This phase will be a long and repetitive cycle. Search teams will be looking for anything of evidentiary value."



Cadaver dogs will assist the search teams in the effort, which will be conducted weekdays from 7:30 a.m. to 6 p.m., McManus said. He estimated more than 20 people will be involved.



Rain would not compromise any evidence but could cause dangerous toxic runoff, which Fire Department and hazardous materials experts would work to contain.



San Antonio police have been working with the FBI and Tempe police. They have been looking at every lead and "chasing them down very, very vigorously," McManus said.



"We are doing everything that we can to try to track down baby Gabriel, and hopefully we will find that baby alive."
Who said that she had killed him and that she gave him away? What cities police are excavating the landfill ? What say his mother? Who has been missing since December 26? Who  killed the baby? Who is excavating landfill?
Elizabeth Johnson, San Antonio She told Gabriel's father she killed the boy and has also said she gave him away to a couple in San Antonio, 8-month-old boy Elizabeth Johnson, Investigators
question: Who said that she had killed him and that she gave him away?, answer: Elizabeth Johnson, | question: What cities police are excavating the landfill ?, answer: San Antonio | question: What say his mother?, answer: She told Gabriel's father she killed the boy and has also said she gave him away to a couple in San Antonio, | question: Who has been missing since December 26?, answer: 8-month-old boy | question: Who  killed the baby?, answer: Elizabeth Johnson, | question: Who is excavating landfill?, answer: Investigators