After about 20 minutes, the men left with Briant, ordering his mother and the other children not to call police, the department said in a statement.
"They grabbed my kid, told me 'I'm going to take the kid to Mexico and I'm going to kill him,'" the distraught Millan said in Spanish to reporters gathered in her front yard Monday night. "I said 'Why? Don't take my kid,' and he told me to 'turn around, I'm going to shoot you. Stop talking.'"
The motive for the abduction was unknown.
"There has been no demand for ransom, there has been no contact with the family," sheriff's spokeswoman Jodi Miller said Monday.
Miller said authorities along the Mexican border had been put on alert, and FBI investigators were helping in the investigation.
"It's horrifying," sheriff's Lt. Rick Ells said. "I don't think I could impress on you how rare a kidnapping like this is."
Briant's older brother said the men tried to steal the family car.
"Our keys were inside our car, cause we were about to leave," 14-year-old Benjamin Barasa told KNBC-TV. "So they couldn't find them, so they got mad. They took my little brother. He didn't know what was happening so he wasn't crying."
One of the children - an 8-year-old boy - wiggled free from his ties and freed the rest of the family, authorities said. Briant is the youngest of Millan's seven children. Her 16-year-old son was not home and another adult child does not live at home.
The boy's father was at work at the time and the initial investigation pointed to the kidnappers being strangers to the family, Ells said.
The gunmen stole Millan's cell phone, so she had to make the emergency call from a nearby liquor store, Ells said.
No witnesses saw the men's vehicle.
The family lives in a modest, single-story home in a mainly lower-income area abutting the city of San Bernardino, about 60 miles east of Los Angeles.
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