Investigators were searching Monday for a 3-year-old boy kidnapped by two gunmen who broke into his family's home, tied up his mother and four siblings, and stole property, authorities said.

The California Highway Patrol issued an Amber Alert late Sunday for 3-year-old Briant Rodriguez, about nine hours after the family called San Bernardino County authorities.

The men, each carrying a handgun, burst through the front door around 2:30 p.m. and tied up the boy, four of his siblings and their mother, Maria Rosalina Millan. They then ransacked the house and stole money and other property, the county sheriff's department said.

After about 20 minutes, the men left with Briant, telling the still-bound mother and the other children not to call police, the department said in a statement.

A motive for the abduction remained unclear.

"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."

One of the children — an 8-year-old boy — wiggled free from his ties and freed the rest of the family, sheriff's spokeswoman Cindy Beavers said. Briant is the youngest sibling of Millan's seven children. Her 16-year-old boy was not home and another adult child does not live at home. No witnesses saw the men's vehicle.

The boy's father, Raul Rodriguez, was at work at the time and the initial investigation pointed to the kidnappers being strangers to the family, Ells said. There were no emergency responses to the house in the previous 90 days.

"It doesn't look like it's a familial abduction of any sort," Ells said.

Sheriff's spokeswoman Jodi Miller said authorities along the U.S.-Mexico border had been put on alert, and FBI investigators were also helping in the investigation.

The family lives in a modest, single-story home in a mainly lower-income slice of county territory abutting the city of San Bernardino, about 60 miles east of Los Angeles. Journalists photographed a distraught-looking Millan standing outside her home Monday.

Briant is a Hispanic boy 3 feet tall and weighs 40 pounds, with brown eyes and long, curly brown hair. He was wearing a yellow shirt, blue-striped shorts and black sandals when he was taken.