"[5] Jaclyn has been described as a happy, "bubbly" and content child who enjoyed playing with dolls and was popular among her peers at Central Park Elementary School.

[6] Her mother worked within the dietary department at Oak Forest Hospital, and her stepfather as a construction foreman at Rax Erecting Service.

[7] By 1988, the family resided in a one-story ranch-style home on 148th Street in a middle-class section of Midlothian, Illinois, surrounded by several nature reserves.

[10] On the afternoon of September 9, 1988, Cynthia took her two children to a local Kentucky Fried Chicken for dinner while her husband visited a Blue Island bowling alley with some friends.

[12] A search of the house revealed no trace of Jaclyn, although the Dowalibys did note the kitchen door to their property—typically locked at nights—was slightly ajar.

[4] A forensic examination of the house itself yielded few clues, although an examination of the broken basement window revealed an even layer of dust on the windowsill inside the home and no recently deposited hairs or fibers upon this surface,[12] indicating the intruder or intruders had not entered the premises via this method; however, a smudge described as being an "indeterminate mark on the wall" was discovered directly beneath this window.

[12][25] Beyond the damaged window, the child's missing blanket and mild disarray in her bedroom—located directly opposite her parents' room—the house bore few signs of a forced entry, although a forensic examination of Jaclyn's pillow did reveal a human hair described in police reports as originating from a Negroid individual.

[29] Initially, a police spokesman confirmed the likelihood Jaclyn had been the victim of a kidnapping, with her abductor or abductors most likely leaving the house with the child via the kitchen door,[21][n 9] although authorities stated they maintained an open mind as to the actual motive behind her disappearance;[9] however, as no ransom demands were made, the day after Jaclyn's abduction, state and local authorities requested the FBI become involved in the manhunt to locate the child.

[29] In the days immediately following their daughter's abduction, the Dowalibys willingly submitted to any police requests to assist in their ongoing investigation.

[19] At approximately 5:45 p.m. on September 14, 1988, a man named Michael Chatman[31] discovered Jaclyn's body discarded upon a section of wasteland close to a garbage receptacle serving a small apartment complex named the Islander Apartments within the city of Blue Island, approximately 6 mi (9.7 km) from her home.

[29] Chatman would later state that upon parking his vehicle, he had become suspicious of a putrid odor and, upon investigation, noted a head and arm protruding from a purple and white blanket which concealed her entire body beneath the upper torso.

The body was transported to the Cook County Medical Examiner's office, and was formally identified via dental records the following day.

[13] An autopsy conducted by Dr. Robert Stein revealed Jaclyn had been strangled with a two-foot section of twine which was still knotted around her neck.

Furthermore, the advanced decomposition and maggot infestation of her remains led Stein to estimate that Jaclyn had most likely been murdered sometime in the morning of September 10.

The day the child's body was discovered, David was again called into the police station to submit to a polygraph test; on this occasion, he was informed the results were inconclusive.

[19][38] On September 17, following a Mass held within St. Christopher's Church attended by over two hundred mourners,[39] Jaclyn was laid to rest within Saint Mary Catholic Cemetery and Mausoleum in Evergreen Park.

[40] Numerous items were confiscated in this search in addition to the family car—a light blue Chevrolet Malibu,[8] which was subjected to a detailed forensic examination.

[8] Shortly thereafter, the Dowalibys retained two lawyers named Ralph Meczyk and Lawrence Hyman,[41] who instructed the couple to refuse to speak with investigators unless they advised them to.

[43] A grand jury indicted Cynthia and David Dowaliby the following day, attesting that the couple had murdered Jaclyn and attempted to conceal the homicide.

The evidence produced at this hearing was Mann's purported identification of David in the early hours of September 10 close to the garbage receptacle where her body had been discovered four days later, and initial testimony indicating the basement window of the Dowaliby household had been broken from the inside as opposed to the outside.

[45][n 10] The day following the Dowalibys' indictment, a renewed forensic report ruled that concentric breaks and stress marks evident upon the basement window indicated the glass had actually been broken from the outside,[47] with the perpetrator having punctured the pane to reduce shattering sounds before removing several large sections of glass and carefully laying them on the ground.

[48] The prosecutors were Patrick O'Brien and George Velcich; Cynthia was defended by Lawrence Hyman, and David by Ralph Meczyk.

[52] The primary prosecution witness to testify at the Dowalibys' trial was eyewitness Everett Mann, who testified to having witnessed a silhouetted Caucasian person he believed to be a man, with a prominent nasal bridge,[38] sitting inside a parked car from a distance of seventy-five yards at approximately 2 a.m. on September 10 before the vehicle drove away from the scene.

[36] Upon cross-examination, Mann conceded he had originally described the vehicle he had seen as being a "late 1970s model" Chevrolet Malibu, possibly dark blue in color, whereas the Dowalibys owned a light blue 1980 model Chevrolet Malibu, which was markedly different in design to 1970s models of the vehicle;[53] he also conceded the fact that the illumination of the parking lot at this time limited his vision and that he was unable to conclusively determine whether the person was either male or female or black or white, although in rebuttal, the prosecution reiterated to the jury that Mann had picked David's image out of a police lineup.

Shortly prior to the trial's conclusion, Judge Neville conferred privately with both counsels and stated that insufficient evidence existed against Cynthia,[56] but the case against David would continue.

[57][59] Immediately prior to Judge Neville imposing sentence, David again professed his innocence before stating he would always love Jaclyn "forever in my heart.

[64] On January 4, 1993, a spokesman for the Cook County State's Attorney announced that prosecutors were actively reviewing new evidence indicating the possibility that Jaclyn's paternal uncle Timothy Randall Guess—a known mentally ill individual—may have been the perpetrator of her abduction and murder, although this individual refused to comment whether the investigation had actually been reopened.