The Democratic presidential candidate said the story is accurate except that the camp was Buchenwald, not Auschwitz.
"Senator Obama's family is proud of the service of his grandfather and uncles in World War II - especially the fact that his great uncle was a part of liberating one of the concentration camps at Buchenwald," campaign spokesman Bill Burton said in a statement. "Yesterday he mistakenly referred to Auschwitz instead of Buchenwald in telling of his personal experience of a soldier in his family who served heroically."
Aides said Tuesday that his grandmother's brother, Charlie Payne, helped liberate a Buchenwald sub-camp in April 1945 as part of the 89th Infantry Division.
In a meeting Monday with veterans, Obama discussed the importance of improving treatment for soldiers suffering post-traumatic stress. To illustrate his point, he talked about his own family.
"I had an uncle who was part of the first American troops to go into Auschwitz and liberate the concentration camps. The story in our family was that when he came home, he just went up into the attic and he didn't leave the house for six months," Obama said. "Now, obviously something had really affected him, but at that time there just weren't the kinds of facilities to help somebody work through that kind of pain."
Auschwitz was liberated by Soviet forces as they marched across Poland in January 1945. The U.S. Holocaust Memorial Museum says Americans liberated several death camps in Germany, including Buchenwald, Dachau and Mauthausen.
"On April 4, 1945, the 89th overran Ohrdruf, a subcamp of the Buchenwald concentration camp. Ohrdruf was the first Nazi concentration camp liberated by U.S. troops in Germany," according to the museum. "A week later, on April 12, Generals Dwight D. Eisenhower, George S. Patton, and Omar Bradley visited Ohrdruf to see, firsthand, evidence of Nazi atrocities against concentration camp prisoners."
Obama's mistaken mention of the camp on Monday quickly generated Internet chatter, ranging from puzzlement to outrage. The Republican Party demanded an explanation.
"It was Soviet troops that liberated Auschwitz, so unless his uncle was serving in the Red Army, there's no way Obama's statement yesterday can be true," said Alex Conant, a spokesman for the Republican National Committee.