The inmate who claims to have killed more than 90 women across the United States is now considered to be the deadliest serial killer in country’s history, the Federal Bureau of Investigation has said.

Samuel Little, who has been behind bars since 2012, told investigators last year that he was responsible for about 90 killings between 1970 and 2005. In a news release on Sunday, the FBI announced that federal crime analysts believe all of his confessions are credible, and officials have been able to verify 50 confessions so far.

Investigators also provided new information and details about five cases in Florida, Arkansas, Kentucky, Nevada and Louisiana.

The 79-year-old Little is serving multiple life sentences in California. He says he strangled his 93 victims, nearly all of them women.

Some of his victims were on the margins of society. Many were originally deemed overdoses, or attributed to accidental or undetermined causes. Some bodies were never found.

The FBI provided 30 drawings of some of his victims – color portraits that were drawn by Little himself in prison. They are haunting portraits, mostly of black women.

The agency also provided videos taken during prison interviews with Little. He described a woman he strangled in 1993 – and how he rolled her down a slope on a desolate road.

“I heard a secondary road noise and that meant she was still rolling,” he said.

In another video, he described a victim in New Orleans. “She was pretty. Light-colored, honey-brown skin,” he said with a small smile. “She was tall for a woman. Beautiful shape. And, uh, friendly.”

It was 1982, and they met in a club. She left with him in his Lincoln, and they parked by a bayou.

“That’s the only one that I ever killed by drowning,” he said.

Investigators across the US are still trying to piece together his confessions with unidentified remains and unsolved cases from decades past. In August, he pleaded guilty to murdering four women in Ohio. He was convicted in California of three killings in 2013 and pleaded guilty to another killing last year in Texas.

Samuel Little leaves the courthouse after attending a pre-trial hearing in Odessa, Texas, last year. Photograph: Mark Rogers/AP

Authorities in Knox county, Tennessee, said Monday that a woman named Martha Cunningham was probably a victim of Little’s.

The Knoxville News Sentinel reported in December that a cold case investigator with the Knox county sheriff’s office had identified the victim whom Little called “Martha”. The Knoxville mother’s body was found in a wooded area in eastern Knox county in 1975.

Cunningham’s body was found by a pair of hunters on the afternoon of 18 January 1975. She was bruised and nude from the waist down, her pantyhose and girdle bunched around her knees. Her purse and some of her jewelry were missing. Her body appeared to have been dragged into the woods and dumped behind a pine tree, authorities said at the time.

Despite that evidence, detectives at the time attributed Cunningham’s death to natural causes within a day of the discovery. The medical examiner’s investigative report lists the probable cause of death as “unknown”.

Cunningham was a talented singer and pianist who grew up performing with her parents and her six younger siblings in a gospel group known as the Happy Home Jubilee Singers.

Law enforcement in Tennessee had Little in custody 19 years after Cunningham’s body was found.

Little was convicted of misdemeanor larceny in 1994 in Nashville, and he was sentenced to 90 days in jail, according to Tennessee bureau of investigation criminal records obtained Monday by the Associated Press.

• This article was amended on 10 October 2019 to remove text that was inconsistent with the Guardian’s style guidelines