WASHINGTON -- U.S. President Barack Obama says the botched execution of an Oklahoma inmate highlights significant problems with the death penalty and he's asking his attorney general for a review.

Obama says he found inmate Clayton Lockett's execution Tuesday "deeply troubling."

Lockett convulsed violently during the execution and tried to lift his head after a doctor declared him unconscious. He later died of an apparent heart attack.

Obama said at a news conference Friday that he believes the death penalty is merited in some cases and that Lockett's crimes were heinous. But he says the penalty's application in the U.S. has problems, including racial bias and the eventual exoneration of some death row inmates.

He says he's asking Attorney General Eric Holder for an analysis of the penalty's application.