A woman who shot her son dead then killed herself at a US shooting range said she was the anti-Christ and that she needed to save her son.

In rambling, teary audio recordings left for her boyfriend and authorities, as well as shorter suicide notes, Marie Moore, 44, apologised several times and said repeatedly: "I had to send my son to heaven and myself to hell."

I am the anti-Christ ... Maria Moore points a gun at her 20-year old-son Mitchell. Credit:AP

Authorities said on Wednesday they still had no motive for the murder-suicide that shocked fellow customers and employees at the Shoot Straight gun range in Casselberry, about 16 kilometres north of Orlando, on Sunday.

"We have no clue. I don't even want to begin to speculate," said Deputy Chief Bill McNeil of the Casselberry Police Department, Florida.