Prison officials in Oklahoma say the first of two executions scheduled for tonight have been halted because of problems with the lethal injection. 

The Tulsa World says Clayton Lockett's execution by lethal injection was halted because of "vein failure." 

A reporter for the paper tweets that over 30 minutes into the execution Lockett was still conscious and said something is wrong.

A prison official says Lockett later died of a massive heart attack.  The second execution scheduled for tonight has been postponed.  

These were to be the first executions in Oklahoma using a new drug mixture for lethal injections.