ORANGE, Calif. -- A Southern California woman accused of tossing her 7-month-old son from the fourth floor of a hospital parking garage was arrested after she circled back to the scene four hours later, a police spokesman said Tuesday.

The woman's husband said she has postpartum depression and was hospitalized for it earlier in the summer.

Sonia Hermosillo, 31, of La Habra was arrested Monday night, hours after the baby was thrown over a parking structure railing at Children's Hospital of Orange County, Orange police Sgt. Dan Adams said.

The infant remained in critical condition Tuesday at the University of California, Irvine, Medical Center, which has a trauma unit.

Hermosillo, who could make a first court appearance as early as today, was being held without bail for investigation of attempted murder and is undergoing a psychological evaluation in the jail's medical ward, said Orange County sheriff's spokesman Jim Amormino. She also was on an immigration hold, he said.

Hermosillo's husband, Noe Medina, told The Orange County Register that his wife had been hospitalized for postpartum depression in June, was taking medication and wasn't allowed to be alone with the baby.

"I don't know what happened yesterday," he told the newspaper in a hospital interview.

Sonia Herrera, 40, lives in the apartment directly below and said she knew Hermosillo before the family moved into the complex two months ago. Their children attend the same elementary school.

She said Hermosillo was a normal, loving mother to her daughters and was excited to be pregnant with a son. But after the boy was born, the 31-year-old mother changed suddenly and seemed depressed, Herrera said.

She withdrew and, while bubbly before, hardly talked at all. Herrera became concerned she might have postpartum depression, although she never saw her harm the infant, she said.

"She was different. She was serious," Herrera said. "I asked her many questions, and she just said `Yes' or `No."'

When she saw Hermosillo's picture on the news, she was shocked.

"I just wanted to cry, and I asked, `Why? Why did this happen?"' she said. "I hope everything will be OK for her and for her baby and for her family because the girls are very young."

At a similar apartment complex less than two blocks away in a working-class neighborhood of single-story homes and 1960s-style apartment buildings, former neighbor Carmen Ruiz said she saw nothing wrong with Hermosillo. They spoke almost every day, she said, and Hermosillo was a stay-at-home mother to her two young daughters while her husband worked in construction.

The couple, both immigrants from Mexico City, didn't seem to fight and were friendly with neighbors, she said.

Hermosillo would walk her older daughter, who had just started kindergarten, to school each day.

"She was a nice person. I am in shock," said Cristina Madrigal, who manages the La Habra Terrace Apartments on a side street that joins up with a busy boulevard of strip malls, gas stations and businesses.

Police have not released a motive and the Orange County district attorney's office declined to comment until prosecutors there received the case from detectives, said Farrah Emami, district attorney spokeswoman.

"Our investigators did tell me that they're certain she's the suspect and we're not looking for any other suspects," Adams said.

A witness on the ground saw the baby falling through the air just after 6 p.m. Monday and several people, including a doctor, called 911, said Adams, the police spokesman.

Surveillance video showed Hermosillo's tan Chevrolet Blazer sport utility vehicle with an empty child seat leaving the parking structure a short time later, the sergeant said. The female driver was the only person in the SUV, and the license plate was traced to the Hermosillo home.

Adams said that during the investigation, the La Habra Police Department notified detectives that a man had reported his wife, Sonia Hermosillo, and their son missing.

An Orange police officer driving past Children's Hospital at 10:15 p.m. spotted the Blazer driving on a street about 100 yards from the crime scene and Hermosillo was at the wheel, Adams said.

Police said Tuesday that Hermosillo was cooperating with the investigation. They declined to release any further details.

Court records in Orange County indicate Hermosillo has no major criminal record but pleaded guilty to four traffic violations in La Habra in 2008, including driving without a valid license and having no proof of insurance.

It's unclear whether the family had any connection to Children's Hospital before the incident, Adams said. A hospital spokeswoman referred all calls to police.

The hospital where the incident occurred is about 15 miles south of La Habra, where the couple lives.