Oklahoma execution: Clayton Lockett death witness speaks

A US death row inmate in Oklahoma died of a heart attack after his execution was halted because the lethal injection of three drugs failed to work properly.

The execution of Clayton Lockett, 38, was halted after 20 minutes, when one of his veins ruptured, preventing the drugs from taking full effect.

The execution of a fellow inmate, due two hours later, was postponed.

Courtney Francisco, a reporter with KFOR TV in Oklahoma, described what she was able to witness of the execution before prison officials closed the blinds on the window into the viewing room.

Some may find this audio distressing