Prior to arrival in Twin Peaks
On joining the Federal Bureau of Investigation, Dale Cooper was based at the Bureau offices in Philadelphia, Pennsylvania. It was here Cooper was partnered with the older Windom Earle, a veteran of the Bureau who taught the young Cooper everything he knew about the FBI. (Cooper later refers to his former partner as having a "brilliant" mind). At some point, Cooper would be based under the authority of FBI Bureau Chief Gordon Cole, who dealt with the mysterious 'Blue Rose' cases.
Some time after joining the Bureau, Earle's wife, Caroline was a witness to a federal crime. Earle and Cooper were assigned to protect her, and it was around this time that Cooper began an affair with Caroline, unbeknowst to his partner. However, one night, whilst in Pittsburgh, Cooper let his guard down - and Caroline was murdered by her husband by way of a knife wound to the aorta. Cooper's former partner and mentor had "lost his mind" (and may have indeed been the perpetrator of the federal crime Caroline witnessed), and was subsequently sent to a mental institution. Cooper was absolutely devastated by the loss of the woman he would later refer to as "the love of my life", and swore to never again get involved with someone who was a part of a case he was assigned to.
Three years before his arrival to Twin Peaks, Cooper has a dream involving the plight of the Tibetan people, and revealed to him the deductive technique of the Tibetan method. Deeply moved by what he saw in this dream, it is indicated it was this event that formed the basis of his unconventional methods of investigation.
In February 1988, Dale Cooper reveals to his boss, FBI Regional Bureau Chief Gordon Cole of the portents of a strange dream - whilst at the same time Agent Chester Desmond is investigating the bizarre murder of Theresa Banks in the town of Deer Meadow, north-east Washington. While conducting an experiement involving security camera monitors in the corridor outside his office, Cooper is shocked by the sudden bizarre appearance of Agent Phillip Jeffries in Cole's office - having vanished in the field two years before.
Hurrying towards Cole, and in front of Cooper and his colleague, Agent Albert Rosenfield, Jeffries starts raving in a loud and disturbed manner, referring at one stage to Cooper and yelling “Who do you think this is, there?”. Jeffries refers to names and incidents that are unfamiliar to those listening, before suddenly disappearing into thin air. Suddenly, the three receive a phone call telling of Agent Desmond's sudden disappearance in Deer Meadow.
Following up on this, and sensing a connection, Cooper retraces Desmond's steps through Deer Meadow - but is unable to discover the fate of his colleague or Theresa Banks' murderer. In a recording to Diane, Cooper refers to his deep conviction that the killer will strike again, "but as the old saying goes, who knows where or when?"
Roughly a year later, in 1989, Cooper tells Rosenfield in the Philadelphia offices of how he senses Banks' killer will strike again soon, and that his victim will be a young woman, who has blonde hair, is sexually active, using drugs, and is crying out for help. (And that Rosenfield will help him solve the case). Rosenfield is quick to dismiss Cooper's notion, however, reminding Cooper he is "talking about half the high school girls in America!"