A man has pleaded guilty in Seattle, Washington, to murdering 48 women, making him the worst serial killer in U.S. history. Gary Ridgway will spend his life in prison under a plea agreement that spares him the death penalty.

The 54-year-old truck painter showed little emotion as the prosecutor read the defendant's statement saying he targeted women he thought were prostitutes.

The killings began in Seattle in 1982, and bodies began turning up on the banks of Washington State's Green River. The unknown assailant was dubbed the Green River Killer, although bodies were also found in ravines and near highways.

The murders were thought to have ended in 1984, but Ridgway has admitted to other killings that occurred in 1990 and 1998.

Ridgway allegedly sexually assaulted many victims before strangling them. Most were prostitutes or young runaways, and he says he selected them because he thought they would not be missed.

Ridgway initially pleaded not guilty, but has recently cooperated with police in locating the bodies of victims. In return, prosecutors agreed not to ask for the death penalty. Death penalty supporters have criticized the agreement, but the lead prosecutor says that without it, families of the victims would never have learned the fate of their loved ones.

Ridgway was long considered a prime suspect in the crimes, but it took years before DNA tests would link him conclusively to several victims.