MIAMI, United States (AFP) — A Florida man is to be executed by lethal injection in the southern United States (US) state on Wednesday for the murders of his wife’s sister and her parents.

David Pittman, 63, will be the 12th person executed in Florida this year and the 31st in the United States.

The execution is to be carried out at 6:00 pm (2200 GMT) at the Florida State Prison.

Pittman was sentenced to death in 1991 for the 1990 murders of Bonnie Knowles, 21, the sister of his estranged wife, and her parents, Clarence Knowles, 60, and Barbara Knowles, 50.

They were stabbed to death and their home then set on fire.

Pittman’s lawyers have sought a stay of execution, claiming he is intellectually disabled with an IQ of just 70, but the appeals have been denied.

There have been 30 executions in the United States this year, the most since 2014 when 35 inmates were put to death.

Florida has carried out the most executions — 11 — followed by South Carolina and Texas with four each.

Twenty-five of this year’s executions have been carried out by lethal injection, two by firing squad and three by nitrogen hypoxia, which involves pumping nitrogen gas into a face mask, causing the prisoner to suffocate.

The use of nitrogen gas as a method of capital punishment has been denounced by United Nations experts as cruel and inhumane.

The death penalty has been abolished in 23 of the 50 US states, while three others — California, Oregon and Pennsylvania — have moratoriums in place.

President Donald Trump is a proponent of capital punishment, and on his first day in office, called for an expansion of its use “for the vilest crimes.”