Samuel Little will be confirmed as one of, if not the most, prolific serial killers in US history.

A 78-year-old drifter in prison in Texas has confessed to 90 murders and is being investigated as possibly the most prolific serial killer in US history.

Samuel Little preyed mainly on drug addicts and prostitutes during a decades-long murder spree that stretched from coast to coast, the Federal Bureau of Investigation (FBI) said in a report.

Little, a 6ft 3in (1.9m) former boxer also known as Samuel McDowell, was arrested at a homeless shelter in Kentucky in 2012 and extradited to California to face drug charges.

Once there, DNA evidence linked him to three cold cases and Little was convicted in 2014 of murdering three women in Los Angeles between 1987 and 1989.

All three had been beaten and strangled.

Sentenced to life in prison, Little was transferred to Texas in connection with the investigation into another murder.

Bobby Bland, district attorney of Ector County where Little is being held, said he eventually confessed to the 1994 murder of Denise Christie Brothers in Odessa, Texas.

And after a Texas Ranger named James Holland gained his trust, Little began confessing to dozens of other murders committed between 1970 and 2005, Bland said.

FBI crime analyst Christina Palazzolo said during the course of an interview in May 2018, Little "went through city and state and gave Ranger Holland the number of people he killed in each place.

"Jackson, Mississippi - one; Cincinnati, Ohio - one; Phoenix, Arizona - three; Las Vegas, Nevada-one..." Palazzolo said.

A total of 90 murders in all, of which law enforcement has so far verified 34 killings.

"Little will be confirmed as one of, if not the most, prolific serial killers in US history," Bland said in a statement.

The deadliest known US serial killer is believed to be Gary Ridgway, the so-called "Green River Killer" convicted of 49 murders who is serving a life sentence in Washington state.

The FBI said it was working with the Department of Justice, Texas Rangers and dozens of state and local agencies to match Little's confessions to unsolved murders across the country.

Not classified as homicides

According to the FBI, Little "remembers his victims and the killings in great detail" but is "less reliable, however, when it comes to remembering dates."

Because his victims were mostly drug addicts and prostitutes, in some cases the women were never identified and their deaths were not investigated.

"Little's method of killing also didn't always leave obvious signs that the death was a homicide," the FBI said.

"The one-time competitive boxer usually stunned or knocked out his victims with powerful punches and then strangled them," it said.

"With no stab marks or bullet wounds, many of these deaths were not classified as homicides but attributed to drug overdoses, accidents, or natural causes."

Little grew up in Ohio, dropped out of high school and lived a "nomadic life," shoplifting or stealing to buy alcohol and drugs, the FBI said.

His criminal record dates back to 1956 with arrests for shoplifting, fraud, drugs and breaking and entering.

He was accused of murdering women in Mississippi and Florida in the early 1980s but was not convicted.

The FBI said Little is in poor health and is likely to spend the remainder of his days in prison in Texas.

The FBI did not say what ailments he suffers from but The New York Times said he is wheelchair-bound and has heart disease and diabetes.

Sergeant Michael Mongeluzzo, a Florida detective, told the Times that he had asked Little during an interrogation how he managed to avoid arrest for all these years.

"I can go into my world and do what I want to do," Mongeluzzo recalled Little as saying. "I won't go into your world."