Billings, Mont. (AP) - The only U.S. flag not captured or lost during George Armstrong Custer's Last Stand at the Battle of Little Bighorn in southeastern Montana sold at auction Friday for $2.2 million.

The buyer was identified by the New York auction house Sotheby's as an American private collector. Frayed, torn, and with possible bloodstains, the flag had been valued before its sale at up to $5 million.

The 7th U.S. Cavalry flag - known as a "guidon" for its swallow-tailed shape - had been the property of the Detroit Institute of Arts, which paid just $54 for it in 1895.

"We'll be using the proceeds to strengthen our collection of Native American art, which has a rather nice irony to it, I think," said the Detroit museum's director, Graham Beal.

Custer and more than 200 troopers were massacred by Lakota Sioux and Northern Cheyenne warriors in the infamous 1876 battle. Of the five guidons carried by Custer's battalion, only one was immediately recovered, from beneath the body of a fallen trooper.

And while Custer's reputation has risen and fallen over the years - once considered a hero, he's regarded by some contemporary scholars as an inept leader and savage American Indian killer - the guidon has emerged as the stuff of legend.

The other flags were believed captured by the victorious Indians.

The recovered flag later became known as the Culbertson Guidon, after the member of the burial party who recovered it, Sgt. Ferdinand Culbertson. Made of silk, it measures 33 inches by 27 inches and features 34 gold stars.

For most of the last century the flag was hidden from public view, kept in storage first at the museum and later, after a period on display in Montana, in a National Park Service facility in Harpers Ferry, W.Va., according to Beal, the museum director.