A harrowing video shows a little boy precariously balancing on a narrow, slanted ledge of a second story window.

Onlookers couldn't believe it when they saw the child, who was barefoot and wearing only underwear, on the ledge of a downtown apartment block in Vallejo, California, Tuesday evening. Behind him, a little girl stood at the window in a diaper.

"Oh my God, where are the police? I don't know what to do!" one witness, Marianne Kearney-Brown, is heard saying in video she recorded from her office window across the street. She also called 911.

Passersby were unable to open doors to the building, preventing them from reaching the children.

The boy managed to keep his balance by hanging onto a window handle while his sister appeared at the window behind him.

Among witnesses was 9-year-old Branson Thrower. He told his mother, who also called 911.

"I was saying, 'Don't jump, go back in! Do not jump!'" Branson told Inside Edition. "The little boy was laughing and smiling. This was no joke ... I wanted him to get back inside and be safe. I thought he was going to jump and die."

He added, "Where are the parents? Is there a babysitter to protect these kids?"

By the time police arrived 30 minutes later, the child had gone back into the apartment. Police in Vallejo have since said the emergency call was not properly prioritized.

"This type of call warrants an immediate response," Vallejo Police Lt. Jason Potts told CBS News. "We made some mistakes and we're going to fix it to make sure it doesn't happen again,"

Police said they have reached out to the children's parents and are investigating.

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