According to sources close to the investigation Gary Ridgway, the so-called Green River killer, will admit murdering 42 of the 49 women on the investigator's list, and another six who are not on the list.
The murderer who preyed on sex workers, drug addicts and the homeless in the early 1980s gained the name from the waterway where the first bodies of his victims were found, in the southern suburbs of Seattle, in 1982.
In many cases he had sex with his victims, strangled them and dumped their bodies along the river or in isolated wooded areas. The murders stopped abruptly in 1984.
Mr Ridgway, 54, was arrested almost two years ago and charged with seven murders, on the basis of DNA evidence and microscopic paint particles allegedly linking him to most of the deaths.
It appears that his killing spree continued much longer than had previously been thought, since at least one of the further six deaths he is prepared to acknowledge took place in 1998.
Mr Ridgway, described by acquaintances as a friendly, somewhat overbearing man who likes to read the Bible, was first arrested in a prostitution sting in 1982. Two months later the Green River killings began. A few months later he was arrested for loitering for the purpose of prostitution. In both instances he was found guilty.
Always a key suspect, he was accused of choking one of the sex workers who got away. At the time the police found no physical evidence to tie him to the crime, but they took a saliva sample, which later proved to be crucial.
A breakthrough came when detectives working with forensic scientists at Washington State University's crime laboratory linked his DNA to three victims. He began cooperating with the authorities months ago, a source said, and led them to four sets of remains in the summer.
"He deserves the death penalty, but what would be the point?" Tim Meehan, whose pregnant sister Mary was found dead in 1983, said. "Twenty years from now, when he'd actually be put to death, he'd be in his mid-70s. At least now the families have an opportunity to have answers.
"Closure is well worth the trade-off."