The Post reported that Walter Reed officials did not respond to requests for comment Tuesday, but that in an e-mailed statement published Monday on RollingStone.com, spokesman Steve Sanderson said the medical center received the request for participation by Baez just two days before the concert.

"These additional requirements were not in the agreement/contract and would have required a modification," Sanderson told the magazine's Web site.

Baez's manager, Mark Spector, told the Post that Mellencamp's management invited Baez to perform in March and handled all the arrangements. The Post said Mellencamp's manager, Randy Hoffman, did not return calls requesting comment and that Mellencamp's publicist said the singer was ill Tuesday and unavailable.

But Mellencamp earlier told RollingStone.com: "They didn't give me a reason why she couldn't come. We asked why and they said, 'She can't fit here, period.' "