The southern US state of Texas on Wednesday executed an inmate who had dropped all appeals and asked to be put to death for murdering his neighbors 13 years ago.

The authorities carried out the scheduled execution of Barney Fuller, 58, in the state with the nation's busiest death chamber, amid a decline in the use of capital punishment across the country.

"I don't have anything to say, you can proceed Warden Jones," Fuller said in his last statement before Texas Department of Criminal Justice officials administered a lethal injection -- the state's first execution in six months.

Fuller was pronounced deceased at 7:01 pm (2401 GMT), becoming the 16th prisoner to be executed in the United States this year.

He was convicted in the grisly murders of his neighbors, Nathan and Annette Copeland, in May 2003.

The couple had filed a complaint after receiving a threatening phone call from Fuller, a weapons enthusiast, the indictment against him read.

Enraged about an order to appear in court, Fuller drank all night before going to their home armed with several guns.

He used a semi-automatic rifle to spray the front of the Copeland home with some 60 bullets, reloading three times.

- Two survivors -
Fuller then forced his way inside. He shot the husband in the bedroom, and the wife in a bathroom where she was hiding, court documents said.

Just before being shot, Annette Copeland had dialed 911 and the operator clearly heard Fuller say "Party's over, bitch!"

Fuller then twice shot the couple's 14-year-old son, who survived. He was not able to find their 10-year-old daughter because he couldn't locate the light switch in her room.

Fuller gave up all appeals last year, saying the living conditions on death row were unbearable.

He was the 143rd death row inmate to "volunteer" for execution since 1977, according to the Death Penalty Information Center website.

- Support for death penalty -
The low number of executions carried out so far this year reflects a combination of factors: a shortage of the drugs used in lethal injections, a decline in the number of capital punishment verdicts and indictments, and an uptick in appeals which raise the final cost of executions.

Support for the death penalty in the United States has fallen to its lowest level in 40 years, a study published last week by Pew Research Center found.

Most notably, a majority (51 percent) of Americans aged 18 to 29 say they are against the death penalty.

Among the population as a whole, 49 percent of Americans still support capital punishment for people convicted of murder, versus 42 percent who are opposed.

The United States executed 28 inmates in 2015, the lowest figure in 24 years and far from the high of 98 executed in 1999.

Death penalty opponents hope the punishment will become even more rare if Democrat Hillary Clinton wins the White House in November and nominates a liberal justice to fill a vacancy on the divided Supreme Court.