As I understand it, the official records have never been released, and cannot be without the consent of the subject. Blatty apparently saw some brief notes written up by an attending priest (sometimes referred to as the "diary") and spoke to a couple of people involved in the case. Most of the story, he simply made up. (He was an English major at Georgetown at the time of the case, and hoped to make a career as a creative writer/novelist/whatever. He read about the case in the newspaper and saw a story idea.) The really lurid stuff in the book and the movie is pure fiction. Independently of Blatty, there is also gossip and hearsay from people who were around at the time. The case is still part of neighborhood lore; people who grew up in Cottage City, Mount Ranier, and Bladensburg -- and I've met several -- know about it from local legend (which is now commingled with movie lore in local "memory"). But the official medical and canonical records, whatever they may have been, have not been opened.
This intrigues me because this case occurred in the DC area. We are not dealing here with Father O'Gullible in the back of beyond improvising in something way above his pay grade. Catholic University, with its collection of assorted seminaries, was less than two miles away. It would strike me as extraordinary for such a case to happen in CU's backyard and for no one to drive over to evaluate it. (Or more likely, to be called in to consult.) The first exorcism was attempted at Georgetown University Hospital, so the medical and psychiatric staff as well as the exorcists would have had a look at the patient as well. The GU exorcism failed, so the mother took the kid to St. Louis, where a second, prolonged exorcism ultimately succeeded. Since the Catholic Church has a protocol for authorizing exorcisms, this means that a second set of eyes in a different diocese and reporting to a different bishop also signed off.
The professional skeptics will argue that (1) mentally ill people often produce quite dramatic symptoms, with no supernatural causes, that can fool unwary observers and that (2) some patients deliberately trick observers, and that some of them are very, very clever in how they do so. I am sure that both these things are true. In this case, however, the patient was a 12 year old boy when the symptoms began. He was 13 at the time of the second exorcism. He was in the backyard of Catholic Central; he was seen by the staff of a well-regarded teaching hospital; and he was evaluated independently by an independent team in St. Louis. The Catholics may have been much more casual about exorcisms then than now, or perhaps they were just careless in this case. But this would seem to be a case where the official records would be worth a good look, if they still exist.
As an aside, the boy's family was Lutheran, not Catholic. The dad apparently never believed he was possessed; it was the (perhaps hysterical) mother who was convinced. He had been expelled from Bladensburg Jr. High for bad behavior; I've never seen any investigation of what the teachers, principal, and school district officials may have thought. The family consulted first with their doctor, who was stumped, and their Lutheran pastor, who was stumped. The Lutheran pastor referred them to the priest at the neighborhood Catholic Church on the theory that the Catholics were the experts on exorcism. That's a referral I'd like to know more about as well; one would think a Lutheran pastor would call in a Lutheran consultant first, before defaulting to the papists down the street.
Since you're wondering, the famous Georgetown steps had nothing to do with the case. Blatty threw them in as an atmospheric touch to an entirely fictional murder. No one died in the real case.
My father knew Fr. Hughes, the local parish priest in Mt. Rainier, who was very young, and had no experience doing exorcisms. Archbishop O’Boyle, also young and naïve, gave him permission to perform an exorcism, which was very imprudent. Hughes is the priest who was gouged in the leg by the boy, using a bed spring. He nearly bled to death, and limped the rest of his life. He described being at the house; dinner was on the table, steaming hot, and before anyone went into the dining room, the table rose in the air and tipped upside down.