A convicted killer who fatally shot his girlfriend and her daughters in their Tennessee home nearly 40 years ago groaned in pain as he was executed today.

Byron Black died at 10.43am Tuesday, roughly 10 minutes after the lethal injection started.

'It's hurting so bad,' Black told a spiritual advisor at 10.33am as his execution began. He reportedly did not deliver a formal last statement.

The murderer, 69, looked around the room and could be heard sighing and breathing heavily, witnesses to his execution allege.

Black was convicted in the 1988 shooting deaths of his girlfriend Angela Clay, 29, and her two daughters, Latoya Clay, 9, and Lakeisha Clay, 6.