Authorities credited Celia McGinty of Moscow, Idaho, with foiling a plot to bomb Chippewa Valley High School outside Detroit.
Police said a search of 17-year-old Andrew Osantowski's home last week turned up instructions for making a bomb and videotapes of him with assault weapons. Osantowski was arrested Thursday; his father and a family friend also were charged.
McGinty met Osantowski online in a music chat room three weeks ago. She said Monday on ABC that the boy was very specific about how he would take revenge on teachers and schoolmates, including that he planned to kill a police liaison officer.
"He told me where he had his weapons," she said. "He gave me his name and address. Who would do that?"
Police said Osantowski alerted her father, George, who heads the cyber crime unit for the Washington State University police.
Osantowski has been jailed on more than $1 million bond on 10 felony charges, including threatening an act of terrorism, and could face up to 20 years in prison. A judge entered a plea of not guilty on his behalf.
Items found in his home included weapons and ammunition, Nazi flags and books about white supremacy and Adolf Hitler, police said.
Marvin Osantowski, 52, the boy's father, was charged with concealing stolen firearms and pleaded not guilty. Bond was set at $500,000.
Dominic Queentry, 33, a family friend, provided the teen with bomb-making instructions, police said. Investigators said they found explosives, weapons and drugs in his home. He was charged with possessing explosives and other counts and pleaded not guilty. Bond was set at $100,000.