Story highlights The FBI says the boys' mother took them to Mexico in August

Their father, David Cook, had custody of the children

(CNN) Two brothers allegedly kidnapped by their biological mother and taken to Mexico six months ago have been reunited with their father.

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."

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