FBI agents are searching for a rare form of bank robber: A lone woman who ripped off an Arlington County Chevy Chase Bank last month.

In 2006, the most recent year for which the FBI has a full statistics, only 6 percent of the nearly 9,000 bank robberies in the United States were committed by women. And FBI officials said, even when women do rob banks, it’s rarely alone, although there are no statistics separating women who act alone or with a counterpart.