Welch was found in Tacoma's Puget Park in March 1986, Bastian five months later in Point Defiance Park. Both had been raped and murdered.
At first, police thought both murders were the work of the same man, until 2013, when a re-examination of evidence proved there were actually two different killers.
Earlier this year, Jennifer Bastian's suspected killer was also arrested.
That suspect was detained out of state and taken to Washington for prosecution. Officials with the Illinois State Police said they helped apprehend the suspect in Bastian's killing, identified as Robert Washburn.
In court documents, Pierce County prosecuting attorney Jared Ausserer said Washburn first became a suspect when he called police in May of 1986 about a composite sketch released of a suspect in the murder of Welch.
Washburn called police after the suspect sketch was released, saying he saw a similar-looking man while jogging in Point Defiance Park.
He told police he jogged in the park as often as twice a day, Ausserer said.
In 1986, a special task force was formed to investigate the murders of Welch and Bastian.
"There are remarkable similarities (in the murders of Welch and Bastian)," KIRO 7 reporter Karen O'Leary said on-air that year. "She and Jennifer were about the same age. Both were riding bikes when they disappeared. Both were found in north Tacoma parks."