The Black Hawk Down star checked into the hospital on Sunday after a "flare-up" of an ongoing stomach problem that first affected him when he was performing on London's West End stage last year.
Hartnett's publicist reveals the actor has undergone tests all week - but doctors have yet to properly diagnose his condition.
Susan Patricola tells Usmagazine.com. "He has had a lot of testing. They released him, and they are trying to figure out what it might be. He is fine."
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