US executions are meant to be clinical and humane, but for some they end up resembling medieval torture, complete with the smell of burning flesh, screams, and scenes so gruesome that witnesses faint.

"We put animals to death more humanely," reporter Carla McClain said of a 1992 execution she witnessed, in which Donald Eugene Harding writhed and thrashed in an Arizona gas chamber for over 10 minutes before dying.

Last month, Romell Brown became only the second man to leave a US execution chamber alive, after 18 failed attempts to administer the lethal injection.

Authorities in Ohio decided to halt his execution after officials spent two hours trying to inject him with lethal chemicals.

Many of those executed in the United States in the last 25 years were not so lucky, suffering through executions in which flesh caught on fire, blood saturated shirts, and witnesses watched and listened as the condemned convulsed and screamed with pain.

In 1999, Florida Supreme Court Justice Leander Shaw reacted with horror to pictures of Allen Lee Davis, who was put to death by electric chair.

"The color photos of Davis depict a man who -- for all appearances -- was brutally tortured to death by the citizens of Florida," Shaw wrote.

Davis had been strapped into an electric chair especially designed to fit his 350-pound frame. As he was electrocuted, but before he was pronounced dead, blood poured from his mouth, soaking his white shirt and oozing through the buckle holes of the strap holding him down.

Michael Radelet, a professor at the University of Colorado, worked with the Death Penalty Information Center to collect testimony on more than 40 botched instances from the witnesses required to be present at executions.

Horror stories have emerged about all the execution methods commonly used in the United States, including the electric chair, lethal injection and gas chamber, with most of the disasters due to human error.

In 1983 in Alabama, a first jolt of electricity caused the electrode attached to John Evans' leg to catch fire. Smoke and sparks also came from under the hood placed over his head, near where an electrode was strapped to his left temple.

A second jolt was administered, but despite the smoke and smell of burning flesh, doctors discovered Evans' heart was still beating and applied a third jolt that finally killed him after 14 minutes.

Two years later, in Indiana, William Vandiver received five separate jolts of electricity over the course of 17 minutes before his heart stopped.

Jesse Joseph Tafero was sentenced to death by electric chair in Florida in 1990, but a synthetic sponge that was used during his execution caught fire, causing six-inch flames to erupt from his head.

Sentenced to death by gas chamber in Mississippi in 1983, Jimmy Lee Gray had the misfortune to be put to death by an executioner who later admitted he was drunk. Gray's gasps and moans so horrified observers that the witness room was cleared by officials.

In recent years, several lawsuits have challenged the lethal injection as "cruel," but it continues to be used by most US states practicing the death penalty and the Supreme Court upheld its constitutionality in 2008.

But for Bennie Demps, who spent 33 minutes of agony as execution technicians tried to find a back-up vein that could support an alternate intravenous drip in case the first one failed, the pain was excruciating.

"They butchered me back there. I was in a lot of pain. They cut me in the groin, they cut me in the leg. I was bleeding profusely. This is not an execution, it is murder," he said in his final statement.

In Angel Diaz's case, in Florida in 2006, a single dose of the lethal cocktails that anesthetize, paralyze and then stop the recipient's heart was not enough. The first injection went through his vein and out the other side, dispersing the chemicals into his muscles, forcing a second dose to be given.

At times, the scenes have been gruesome enough to physically affect observers. In 1989, in Texas, which holds the record for the most US executions, a male witness fainted after watching Stephen McCoy's violent writhing.

Some of the most recent horror stories come from Ohio, where Broom's execution was halted.

"It don't work! It don't work," yelled a sobbing Joseph Clark in May 2006, as the vein that executioners had worked 22 minutes to find collapsed while the chemicals were being administered.

A year later, Ohio authorities took two hours to successfully find veins and administer Christopher Newton the lethal injection. The process took so long, he was authorized to take a bathroom break.

The only other person to have survived execution in the United States was young black man named Willie Francis who survived a Louisiana electric chair in the 1940s. He was later put to death on a second attempt.