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 in the afternoon and tied up the boy, four of his siblings and their mother, Maria Rosalina Millan, 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.
Millan, distraught and pleading for her son's return, told CBS News affiliate KCAL on Monday that the last thing the gunmen told her before they fled was that they were going to take her son to Mexico, and kill him. (Click here for KCAL's coverage.)
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, authorities said. Briant is the youngest 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 assailants stole Millan's cell phone, so she had to make the emergency call from a nearby liquor store, Ells said.
"The clerk said she was real upset, she seemed appropriately upset," he said.
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. 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.
Television footage showed a distraught-looking Millan standing outside her home on Monday, comforted by a young girl. An attempt to locate the family's home telephone number was not successful.
Sheriff's spokeswoman Jodi Miller said authorities along the Mexican 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.
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.