HELENA, Mont. (AP) - A secretive conservative group may simply vanish after spending four years dumping candidate attack ads in Montana elections and leading high-profile challenges to state campaign laws.

American Tradition Partnership is registered as a tax-exempt 501(c)(4) nonprofit social-welfare organization. Attorney James Brown said Tuesday it continues to exist as a legal entity, but the organization is not active.

The group has argued vehemently in lawsuits that its campaign spending was free speech, while fighting just as hard to keep its donors and spending a secret.

But a state judge ruled this month that ATP acted as a political committee that must report its spending and donors. And in December, state officials revealed that a federal investigation into ATP is under way.

ATP's only staffer, executive director Donny Ferguson, resigned this month.