(05-23) 19:06 PDT KNIGHTSEN --

The grandmother of a 4-month-old girl has been arrested for allegedly kidnapping the infant from her home in eastern Contra Costa County and taking a cab back to her home in Los Angeles County, where she tried to pass the baby off as her own, authorities and acquaintances said Monday.

Ericka Gallego, 58, of El Monte sneaked into her son's home on Tule Lane in Knightsen and snatched her granddaughter, Ramy Amadea Gallego, as the baby slept in her bassinet, said Contra Costa County sheriff's Capt. Steve Warne. Then Gallego called a cab to drive them to Southern California, he said.

Ramy's mother discovered her missing at 6 a.m. Sunday. An acquaintance led El Monte police to Gallego's home, where she was arrested and Ramy was found safe, Warne said.

Gallego had told friends she was pregnant and had held a baby shower, said Ivan Jimenez of El Monte, whose father owns the home where Gallego had been living for about a year.

"She was faking a pregnancy," said Jimenez, who saw the baby at his home over the weekend. Jimenez said he knew it couldn't be hers because "she was too old."

Gallego was booked on suspicion of kidnapping at the Los Angeles County Jail, where she is being held in lieu of $150,000 bail. Contra Costa County prosecutors filed a felony charge of kidnaping against Gallego on Monday.

Warne said investigators were "still trying to decipher what the true motive is behind this."

Gallego is the mother of Rudy Gallego, Ramy's father. Rudy Gallego and his wife, Kristin, have three other children.

"Ericka Gallego never had permission to take Ramy from her house," Warne said. "We believe that she snuck into the home without the parents' knowledge."

Rudy Gallego told reporters Monday that he believed his mother loved his daughter and wouldn't harm her. But he said she "just took a very big mental wrong turn. I just hope she gets help."

Ericka Gallego took a bus and cab from El Monte to Knightsen, grabbed the baby and then called a separate cab to take the two back to Los Angeles, Warne said. There was no reason for the cab driver to believe a crime was occurring, he said.

In a Facebook message posted Monday while she was heading to Los Angeles, Kristin Gallego wrote, "This is probably the longest ride to So. Cal that I have ever been on, but it is well worth it. I can't wait for my baby to be in my arms."