How a 2010 move by officials in Ohio triggered a deluge over the Obama administration three years later.
A Crossroads spokesman said Friday that the group was probably subjected to special scrutiny.
In a pointed letter, the Senate Finance Committee outlines 41 specific lines of inquiry.
It's the second major amendment agreed to directly tied to the Boston attacks.
Former Montana governor Brian Schweitzer (D), who Democrats have said is likely to run for that state's open Senate seat, has nonetheless taken a new job as chairman of the board of the Stillwater Mining Company.
Republicans continue to struggle with electing women to office because of the remnants of the party's "old boy network," says the man who leads the party's effort to win state and local elections.