Serial killer Thomas Eugene Creech, 73, who has been convicted of five murders and is suspected of several more, was due to be executed at 10am local time (5pm UK time) in a maximum-security Idaho prison.

Supporters said he had changed and asked for his sentence to be converted to life in prison, arguing he had become kind and supportive while behind bars.

While prosecuting lawyer Jill Longhurst admits he can be polite and friendly with prison staff, she described him as a "psychopath" who lacks remorse and empathy for others.

And his final chance at a last-minute reprieve was denied by the Supreme Court, despite claims his execution should be delayed so a court could weigh Creech's claim prosecutors lied.

A flurry of late appeals against the execution had previously been dismissed by judges in four courts.

What will happen to Creech?

Creech will be asked if he would like a mild sedative to calm him before he is killed.

Officials will then bring him to the execution chamber and strap him to a padded medical table, where volunteers will insert a catheter into one of his veins.

He will get a chance to say any final words and a spiritual adviser will be allowed to pray with him and touch his shoulder - though he cannot hold his hand or make any noise when injection begins.

Creech may also wear a crucifix and his wife will be seated in the witness area, where he can make eye contact with her.

He will then finally be injected with a drug.

Creech's crimes

It will end a 50-year spell in prison, having been originally sentenced to death for shooting dead John Wayne Bradford and Edward Thomas Arnold.

But that sentence was changed to life in prison after the state's sentencing law was found to be unconstitutional.

Then, in 1983, he was sentenced to death for the murder of David Dale Jensen, who was 22, disabled and serving time for a car theft when Creech beat him to death in prison on 13 May 1981.

During Creech's clemency hearing - seeking forgiveness - last month, Jensen's family members described him as a gentle soul who loved hunting and being outdoors.

Jensen's daughter was just four when he died, and she spoke about how painful it was to grow up without a father.

Further suspicions

In addition to the Idaho murders, Creech has been convicted of two killings in 1974 - William Joseph Dean in Oregon and Vivian Grant Robinson in California.

He was also charged with killing Sandra Jane Ramsamooj in Oregon that year, but the charge was later dropped in light of his other murder sentences.

In 1973, Creech was tried for the killing of 70-year-old Paul Schrader in Arizona, but was cleared of the crime.

Authorities still believe him to be responsible for Mr Schrader's death, and say Creech provided information leading them to bodies of two people near Las Vegas and one person in Wyoming.

Late appeals

Creech's supporters had pushed to have his sentence converted to life without parole, saying he was a deeply changed man.

Several years ago he married the mother of a prison officer, and former staff said he was known for writing poetry and expressing gratitude for the work done by the prison team.

Read more:
Florida man executed after being convicted of killing two women in 1996
Man sentenced to death over 1998 murder freed after wrongful conviction

Creech's execution will be the second in the US this year, according to the Death Penalty Information Centre.

The first was in Alabama last month, when Kenneth Eugene Smith controversially became the first to be executed using nitrogen gas.

Alabama officials said the method was humane and predicted death would take a few minutes, but Smith seemed to remain conscious for several minutes and appeared to writhe in agony for at least two minutes.