OKLAHOMA CITY - Oklahoma inmate Clayton Lockett's biological family has issued a statement Monday in response to his execution.

Lockett, who was convicted of rape and murder in 2000, was supposed to die of lethal injection, but instead died of a heart attack 40 minutes after the drug cocktail was injected into his arm. All this comes after months of legal challenges over the drugs used in the lethal injection.

Lockett's biological family issued the following statement Monday in response to Lockett's execution,

"On behalf of Clayton Darrell Lockett, we the family first send our condolences to the Stephanie Nieman family. Justice has been served for you. However, the Department of Corrections and the State of Oklahoma failed to execute Clayton's execution properly. The DOC and the State had strict guidelines they are obligated to follow and they failed miserably. We were prepared for an execution, but not another homicide. Please give the biological family our privacy.

Sincerely,

Clayton's biological family."

Ladonna Hollins, Lockett's step-mother, spoke with News 9 last week. 

Governor Mary Fallin has called for an independent investigation into exactly what went wrong during the execution.