Fay was fatally stabbed in her west suburban Brookfield home in June.
Bond was denied for Bonnie Shelesny, 24, of Arlington Heights. Prosecutors say Shelesny and her boyfriend, Steven Kellmann, allegedly killed Fay to get money for their heroin drug habit.
"The investigation wasn't really complete on Ms. Shelesny. We took Steven Kellmann into custody with the evidence that we had," said Chief Steven Stelter, Brookfield Police Department.
Police say Shelesny confessed to the crime. She's now been charged with first-degree murder, residential burglary and possession of a controlled substance after she was arrested Wednesday outside the place where she worked.
Shelesny's arrest comes six months after Fay was found strangled, stabbed, and beaten to death in her home on June 14, 2010 by officers conducting a well-being check. She was originally taken into custody with Kellmann, but was questioned and then released.
"In June when Mr. Kellmann was arrested, there was sufficient to show his involvement in the murder of Marilyn Fay. Since that time, we have developed more evidence that shows that Bonnie Shelesny was an active participant in this crime and that's why we were able to go back and arrest her now," said Detective James Burdett, Brookfield Police Department.
Shelesny was originally taken into custody for questioning with Kellmann at a motel on Chicago's Southwest Side in June where police found the victim's car keys, credit cards and cell phone. Authorities say surveillance cameras captured the couple trying to use Fay's debit card at a restaurant and later at a hardware store.
Kellmann had previously worked for Fay, who had befriended him while she worked as a part-time librarian. She sometimes employed Kellmann as a handyman and even allowed him to live with her.
"We're happy that the people responsible for this are behind bars," Alex Leon, neighbor.
Police would not discuss the details of the evidence against Shelesny, who has no other criminal record aside from a misdemeanor retail theft arrest in Arlington Heights that happened after the murder.
Kellmann faces first-degree murder, residential burglary and possession of a controlled substance. He is being held on $2 million bond.