For parents of missing children, any scrap of information that could lead to an abductor is precious.

Three years into an excruciating search for her abducted son, Susan Lau got such a tip. Her estranged husband, who had absconded with their 9-year-old from Brooklyn, had apparently filed a tax return claiming the boy as an exemption.

Investigators moved quickly to seek the address where his tax refund had been mailed. But the Internal Revenue Service was not forthcoming.

“They just basically said forget about it,” said Julianne Sylva, a child abduction investigator who is now deputy district attorney in Santa Clara County, Calif.