Scranton Strangler (real name George Howard Skub)) is a nickname for an unseen serial criminal in Scranton.
To celebrate the birth of Cecilia Halpert, Andy purchases a commemorative copy of the day's newspaper. Unfortunately, the headline for the day she was born is "Scranton strangler strikes again." ("The Delivery")
To demonstrate the utility of wearing one's cell phone on one's belt, Dwight pretends to call 911 to report the Scranton Strangler.
Dwight dresses up as the strangler on Halloween and acts like he's strangling Jim.
In Viewing Party, the cold open features a police standoff and car chase against the Scranton Strangler, presumably ending with his capture.
In "Classy Christmas, Part 1" Toby has to take a leave of absence in order to serve jury duty on a "high-profile case." Although he never explicitly states it, he strongly suggests that the case is the trial of the Scranton Strangler through verbal and physical clues. He then returns to the office and is asked about the strangler's appearance. It is here the gender of the strangler is confirmed.
In the episode "Michael's Last Dundies" Toby, whilst receiving his Extreme Repulsiveness Award, uses his acceptance speech as a platform to announce to the audience that he was part of the jury that found the alleged Scranton Strangler guilty and sentenced him to death, but he is "not so sure he's guilty anymore". No more is revealed as Toby is cut off by Dwight, who begins playing "Oh Yeah" by Yello over his acceptance speech.
Dwight implements the "Accountability Booster" which is supposed to increase efficiency and reduce errors by keeping track of mistakes made by the office employees. The office finds that the error checking is far too stringent, they try to access Dwight's computer by guessing his password. Dwight sees this and sneaks up behind them all and suggests that they try "Scranton Strangler 666" as a password attempt.
When Oscar admits that the numbers he handed in to Toby were false and he simply did it because he wanted to frame Kevin, Toby suddenly becomes worried and admits to Oscar that when he was on the Scranton Strangler jury he was pressured to convict him despite believing he may have been innocent (The Boat).
Toby notes that it is the anniversary of the trial of the Scranton Strangler (and reveals his real name). Nellie, who has never heard of him, listens to Toby tell his story of when he was on the jury. Toby reveals how he did his own research on finger printing and began to think that he was innocent (Dwight Christmas).
Toby mentions his suspicions about the strangler trial, at which Nellie expresses irritation at his lack of action. He decides to travel to the prison to confront the strangler about his belief, but when speaking to the strangler, Toby is attacked, leaving him with damaged vocal cords and a neck brace. This confirms that he was the strangler and Toby was wrong in his suspicion that he was innocent. (Moving On)