A mother who abandoned her 7-year-old son at a bustling train station in downtown Los Angeles was arrested after she returned to the station about 19 hours later, police said Thursday.

The boy wasn't able to communicate with police, who said they were trying to determine if he had any "medical issues or developmental issues."

Workers at a cafe in Union Station first saw the boy and a woman eating together Wednesday evening. They began to worry later when he ran into their kitchen alone and they realized the woman was nowhere in sight, said Adrienne Garrett, supervisor at Cafe Crepe.

Garrett said she and other workers looked for the woman to no avail, but she wandered back about 30 minutes later, prompting one worker to tell her that she should be taking care of her child.

"We asked her to come back, but she literally walked out the front door," Garrett said. "She did not even acknowledge anything at all. She was silent and kept walking — and that's when we decided to get security."

Police bought the boy an ice cream and when he still appeared hungry, Garrett said she gave him some chicken fingers, French fries and apple juice.

The mother was captured on surveillance video walking with the boy outside Union Station and was then seen walking away alone.

A family friend watching a television news report about the case recognized the boy and called his family, who contacted police, said Detective Meghan Aguilar, a police spokeswoman. The boy was reunited with family members Thursday afternoon.

A short time later, the boy's biological mother returned to the train station. After officers recognized her from surveillance video, she was taken into custody, Aguilar said.

The woman was booked on suspicion of felony child endangerment and remained jailed on $100,000 bail.

Investigators are still looking into what would have caused the mother to leave her son at the train station and whether she ever tried to contact authorities. She could potentially face child abandonment or child neglect charges, police said.

Garrett said she was heartbroken for the boy when he was left alone.

"I don't understand how anyone in their right mind would do that," she said. "It's a young boy, it's the Fourth of July, and it's Union Station — anything can happen. It's really upsetting she felt comfortable leaving him here."

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Associated Press writer Michael Balsamo contributed to this report.