Florida has put a man to death with an anaesthetic never used before in a US lethal injection, carrying out its first execution in more than 18 months on an inmate convicted of two racially motivated murders.

Authorities said the 53-year-old Mark Asay, the first white man executed in Florida for the killing of a black man, was pronounced dead at 6:22 pm on Thursday at the state prison in Starke. Asay received a three-drug injection that began with the anaesthetic, etomidate.

Though approved by the Florida Supreme Court, etomidate has been criticised by some as being unproven in an execution. Etomidate replaced midazolam, which became harder to acquire after many drug companies began refusing to provide it for executions.

Prosecutors say Asay made racist comments in the 1987 fatal shooting of a 34-year-old black man, Robert Lee Booker. Asay also was convicted of the 1987 murder of 26-year-old Robert McDowell, who was mixed race, white and Hispanic. Asay had hired McDowell, who was dressed as a woman, as a prostitute, and killed him after learning his true gender, prosecutors said.

Asay was asked whether he wanted to make a final statement. “No sir, I do not. Thank you,” he replied.

The execution protocol began at 6:10 pm. About a minute after the first drug was administered, Asay’s feet jerked slightly and his mouth opened. A minute or two later he was motionless and subsequently was pronounced dead by a doctor. Michelle Glady, a spokeswoman for the corrections department, said there was no complication in the procedure and that Asay did not speak during it.

The execution was Florida’s first since the US Supreme Court halted the practice in the state after finding its method for sentencing people to death to be unconstitutional. The high court had earlier rejected Asay’s final appeal without comment.

Asay was the first white man to be executed in Florida for killing a black man. At least 20 black men have been executed for killing white victims since the state reinstated the death penalty in 1976, according to data from the Death Penalty Information Center. A total of 92 Florida inmates had been executed previously in that time period.

The inmate’s spiritual adviser, Norman Smith of Cavalry Chapel in Melbourne, Florida spent two hours with Asay before his execution. He said he admitted spouting racial epithets prior to Booker’s murder, but said he was drunk and angry, not a racist.

“Until I heard that I would’ve never known that this man was tagged as a racist,” said Smith, who is black. Asay, he added, was ready and not conflicted as the execution hour approached.

Etomidate is the first of three drugs administered in Florida’s new execution mixture. It’s followed by rocuronium bromide, a paralytic, and finally, potassium acetate, which stops the heart. It is Florida’s first time using potassium acetate too, which was used in a 2015 execution in Oklahoma by mistake, but has not been used elsewhere, a death penalty expert said.

State corrections officials have defended the choice of etomidate, saying it has been reviewed. The corrections department refused to answer questions from The AP about how it chose etomidate.

Doctors hired by Asay’s attorneys raised questions about etomidate in court declarations, saying there are cases where it had caused pain along with involuntary writhing in patients. But in its opinion allowing the drug to be used, the state’s high court said earlier this month that four expert witnesses demonstrated that Asay “is at small risk of mild to moderate pain.”

Executions in Florida were put on hold for 18 months after the Supreme Court ruled that the old system was unconstitutional because it gave judges, not juries, the power to decide. Since then, Florida’s Legislature passed a law requiring a unanimous jury for death penalty recommendations.

In Asay’s case, jurors recommended death for both murder counts by a 9-3 vote. Even though the new law requires unanimity, Florida’s high court ruled that the US Supreme Court’s ruling did not apply to older cases. Asay was the 24th inmate executed since Governor Rick Scott has taken office, the most under any governor in Florida history.