Gary Ridgway, known as the Green River killer for murdering 49 people over 20 years, was transferred to a Colorado prison so he could be brought out of isolation, newly released documents reveal.

Since his conviction in 2004, the 66-year-old Ridgway has lived in virtual isolation at the Washington State Penitentiary in Walla Walla, where he's serving a life sentence without parole.

Prison documents obtained by The Seattle Times (http://bit.ly/1KSxVU6 ) show Ridgway was easily recognizable and a target of other inmates.

Officials believe he'll be less well known in Colorado and can be placed in the general population, where he'll have more freedom and social contacts.

Ridgway's transfer to the federal prison in Florence, Colorado came to light in June, but the reason for the transfer was a mystery until the documents were released under Washington's public records law.

The move was conducted in May, at a cost of nearly $20,000 for a private plane to fly Ridgway and another, unidentified inmate to the Florence high-security facility, the documents show.

After his arrest in 2001 based on DNA evidence, Ridgway agreed to an unprecedented plea bargain with the King County Prosecuting Attorney's Office, in which he agreed to detail his crimes in exchange for no death penalty.

Prosecutors were eventually able to charge him with 48 murders between 1982 and 1998.

He pleaded guilty in 2011 to a 49th murder. By Ridgway's own count, the number of victims is closer to 70.

Ridgway will not be housed in the so-called "supermax" prison on the same campus, called ADMAX, where the country's most dangerous prisoners are kept in solitary confinement. Residents there include would-be millennium bomber Ahmed Ressam and Unabomber Ted Kaczynski.

State prison officials had wanted Ridgway placed in ADMAX, but the federal Bureau of Prisons determined he could be safely placed at the high-security facility next door.

Washington prison records show Ridgway was a model inmate while housed in the Intensive Management Unit at Walla Walla. He cooperated with his captors and never broke prison rules, according to the documents obtained by The Times.

However, documents tracking his imprisonment indicate that in recent years Ridgway has complained of unspecified mental problems and has been on medication.

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Information from: The Seattle Times, http://www.seattletimes.com