- The FBI says the boys' mother took them to Mexico in August
- Their father, David Cook, had custody of the children
Sage Cook, 15, and Isaac Cook, 9, disappeared in August after visiting their biological mother, Faye Ku, in Lakewood, California, local authorities said in a statement
Before they vanished, the boys lived in Bellevue, Washington, with their father, the FBI said in a statement Friday. Their father, David Cook, got custody of the children after he and Ku divorced seven years ago, police said.
"Authorities located them in safe condition in Mexico's Sinaloa state," the FBI's Seattle Division said.
"This evening, they returned to Washington with their father, David Cook, and stepmother, Helen Cook, after being reunited in Mexico."
Mother left behind a letter
Investigators allege Ku, 41, crossed the border from San Diego to Tijuana, Mexico, with the children on August 29.
"Ms. Ku intentionally left behind personal belongings at her place of residence that would prevent law enforcement from tracking her whereabouts," Lakewood Sheriff's Station said.
"In a prepared letter, she blamed the children's father for trying to control them, and asked him to leave them alone."
The biological mother has been deported from Mexico and taken into U.S. custody, the FBI said. She is expected to appear in the U.S. District Court in Los Angeles on Tuesday.
Authorities said Ku made another attempt to leave the country with the children three years ago, but she was detained before boarding a flight to Taiwan.